Commit | Line | Data |
---|---|---|
0f4ca9d1 JM |
1 | # The Make-A-Lisp Process |
2 | ||
3 | So you want to write a Lisp interpreter? Welcome! | |
4 | ||
5 | The goal of the Make-A-Lisp project is to make it easy to write your | |
6 | own Lisp interpreter without sacrificing those many "Aha!" moments | |
7 | that come from ascending the McCarthy mountain. When you reach the peak | |
8 | of this particular mountain, you will have an interpreter for the mal | |
9 | Lisp language that is powerful enough to be self-hosting, meaning it | |
10 | will be able to run a mal interpreter written in mal itself. | |
11 | ||
12 | So jump right in (er ... start the climb)! | |
13 | ||
dc791440 DM |
14 | - [Pick a language](#pick-a-language) |
15 | - [Getting started](#getting-started) | |
16 | - [General hints](#general-hints) | |
17 | - [The Make-A-Lisp Process](#the-make-a-lisp-process-1) | |
18 | - [Step 0: The REPL](#step-0-the-repl) | |
19 | - [Step 1: Read and Print](#step-1-read-and-print) | |
20 | - [Step 2: Eval](#step-2-eval) | |
21 | - [Step 3: Environments](#step-3-environments) | |
22 | - [Step 4: If Fn Do](#step-4-if-fn-do) | |
23 | - [Step 5: Tail call optimization](#step-5-tail-call-optimization) | |
24 | - [Step 6: Files, Mutation, and Evil](#step-6-files-mutation-and-evil) | |
25 | - [Step 7: Quoting](#step-7-quoting) | |
26 | - [Step 8: Macros](#step-8-macros) | |
27 | - [Step 9: Try](#step-9-try) | |
28 | - [Step A: Metadata, Self-hosting and Interop](#step-a-metadata-self-hosting-and-interop) | |
29 | ||
30 | ||
0f4ca9d1 JM |
31 | ## Pick a language |
32 | ||
33 | You might already have a language in mind that you want to use. | |
34 | Technically speaking, mal can be implemented in any sufficiently | |
dd4020b9 JM |
35 | complete programming language (i.e. Turing complete), however, there |
36 | are a few language features that can make the task MUCH easier. Here | |
37 | are some of them in rough order of importance: | |
0f4ca9d1 JM |
38 | |
39 | * A sequential compound data structure (e.g. arrays, lists, | |
40 | vectors, etc) | |
41 | * An associative compound data structure (e.g. a dictionary, | |
42 | hash-map, associative array, etc) | |
43 | * Function references (first class functions, function pointers, | |
44 | etc) | |
45 | * Real exception handling (try/catch, raise, throw, etc) | |
46 | * Variable argument functions (variadic, var args, splats, apply, etc) | |
47 | * Function closures | |
48 | * PCRE regular expressions | |
49 | ||
50 | In addition, the following will make your task especially easy: | |
51 | ||
bf518367 JM |
52 | * Dynamic typing / boxed types (specifically, the ability to store |
53 | different data types in the sequential and associative structures | |
54 | and the language keeps track of the type for you) | |
0f4ca9d1 JM |
55 | * Compound data types support arbitrary runtime "hidden" data |
56 | (metadata, metatables, dynamic fields attributes) | |
57 | ||
58 | Here are some examples of languages that have all of the above | |
59 | features: JavaScript, Ruby, Python, Lua, R, Clojure. | |
60 | ||
cf70df1f JM |
61 | Michael Fogus has some great blog posts on interesting but less well |
62 | known languages and many of the languages on his lists do not yet have | |
63 | any mal implementations: | |
64 | * http://blog.fogus.me/2011/08/14/perlis-languages/ | |
65 | * http://blog.fogus.me/2011/10/18/programming-language-development-the-past-5-years/ | |
66 | ||
0f4ca9d1 JM |
67 | Many of the most popular languages already have Mal implementations. |
68 | However, this should not discourage you from creating your own | |
69 | implementation in a language that already has one. However, if you go | |
70 | this route, I suggest you avoid referring to the existing | |
71 | implementations (i.e. "cheating") to maximize your learning experience | |
72 | instead of just borrowing mine. On the other hand, if your goal is to | |
73 | add new implementations to mal as efficiently as possible, then you | |
74 | SHOULD find the most similar target language implementation and refer | |
75 | to it frequently. | |
76 | ||
bf518367 JM |
77 | If you want a fairly long list of programming languages with an |
78 | approximate measure of popularity, try the [Programming Language | |
79 | Popularity Chart](http://langpop.corger.nl/) | |
80 | ||
0f4ca9d1 JM |
81 | |
82 | ## Getting started | |
83 | ||
84 | * Install your chosen language interpreter/compiler, language package | |
85 | manager and build tools (if applicable) | |
86 | ||
87 | * Fork the mal repository on github and then clone your forked | |
88 | repository: | |
89 | ``` | |
90 | git clone git@github.com:YOUR_NAME/mal.git | |
91 | cd mal | |
92 | ``` | |
93 | ||
fc03712f EC |
94 | * Make a new directory for your implementation. For example, if your |
95 | language is called "quux": | |
0f4ca9d1 JM |
96 | ``` |
97 | mkdir quux | |
98 | ``` | |
99 | ||
100 | * Modify the top level Makefile to allow the tests to be run against | |
101 | your implementation. For example, if your language is named "quux" | |
102 | and uses "qx" as the file extension, then make the following | |
103 | 3 modifications to Makefile: | |
104 | ``` | |
105 | IMPLS = ... quux ... | |
106 | ... | |
107 | quux_STEP_TO_PROG = mylang/$($(1)).qx | |
20e8dea0 JM |
108 | ``` |
109 | ||
110 | * Add a "run" script to you implementation directory that listens to | |
111 | the "STEP" environment variable for the implementation step to run | |
e1b080b4 JL |
112 | and defaults to "stepA_mal". Make sure the run script has the |
113 | executable file permission set (or else the test runner might fail with a | |
114 | permission denied error message). The following are examples of "run" | |
20e8dea0 JM |
115 | scripts for a compiled language and an interpreted language (where |
116 | the interpreter is named "quux"): | |
117 | ||
118 | ``` | |
119 | #!/bin/bash | |
120 | exec $(dirname $0)/${STEP:-stepA_mal} "${@}" | |
121 | ``` | |
122 | ||
123 | ``` | |
124 | #!/bin/bash | |
125 | exec quux $(dirname $0)/${STEP:-stepA_mal}.qx "${@}" | |
0f4ca9d1 JM |
126 | ``` |
127 | ||
128 | This allows you to run tests against your implementation like this: | |
129 | ``` | |
e5737b08 | 130 | make "test^quux^stepX" |
0f4ca9d1 JM |
131 | ``` |
132 | ||
a15f76bd | 133 | If your implementation language is a compiled language, then you |
396d869e | 134 | should also add a Makefile at the top level of your implementation |
a15f76bd JM |
135 | directory. This Makefile will define how to build the files pointed to |
136 | by the quux_STEP_TO_PROG macro. The top-level Makefile will attempt to | |
137 | build those targets before running tests. If it is a scripting | |
396d869e JM |
138 | language/uncompiled, then no Makefile is necessary because |
139 | quux_STEP_TO_PROG will point to a source file that already exists and | |
140 | does not need to be compiled/built. | |
141 | ||
0f4ca9d1 JM |
142 | |
143 | ## General hints | |
144 | ||
145 | Stackoverflow and Google are your best friends. Modern polyglot | |
146 | developers do not memorize dozens of programming languages. Instead, | |
147 | they learn the peculiar terminology used with each language and then | |
148 | use this to search for their answers. | |
149 | ||
150 | Here are some other resources where multiple languages are | |
151 | compared/described: | |
152 | * http://learnxinyminutes.com/ | |
153 | * http://hyperpolyglot.org/ | |
154 | * http://rosettacode.org/ | |
334a71b6 | 155 | * http://rigaux.org/language-study/syntax-across-languages/ |
0f4ca9d1 JM |
156 | |
157 | Do not let yourself be bogged down by specific problems. While the | |
158 | make-a-lisp process is structured as a series of steps, the reality is | |
159 | that building a lisp interpreter is more like a branching tree. If you | |
160 | get stuck on tail call optimization, or hash-maps, move on to other | |
161 | things. You will often have a stroke of inspiration for a problem as | |
162 | you work through other functionality. I have tried to structure this | |
45a8b3ca JM |
163 | guide and the tests to make clear which things can be deferred until |
164 | later. | |
165 | ||
166 | An aside on deferrable/optional bits: when you run the tests for | |
167 | a given step, the last tests are often marked with an "optional" | |
168 | header. This indicates that these are tests for functionality that is | |
169 | not critical to finish a basic mal implementation. Many of the steps | |
170 | in this process guide have a "Deferrable" section, however, it is not | |
171 | quite the same meaning. Those sections include the functionality that | |
172 | is marked as optional in the tests, but they also include | |
0f4ca9d1 JM |
173 | functionality that becomes mandatory at a later step. In other words, |
174 | this is a "make your own Lisp adventure". | |
175 | ||
176 | Use test driven development. Each step of the make-a-lisp process has | |
177 | a bunch of tests associated with it and there is an easy script to run | |
178 | all the tests for a specific step in the process. Pick a failing test, | |
179 | fix it, repeat until all the tests for that step pass. | |
180 | ||
5c052692 WH |
181 | ## Reference Code |
182 | ||
0f4ca9d1 | 183 | The `process` directory contains abbreviated pseudocode and |
a15f76bd | 184 | architecture diagrams for each step of the make-a-lisp process. Use |
0f4ca9d1 | 185 | a textual diff/comparison tool to compare the previous pseudocode step |
a15f76bd JM |
186 | with the one you are working on. The architecture diagram images have |
187 | changes from the previous step highlighted in red. There is also | |
c1ef46cb JM |
188 | a concise |
189 | [cheatsheet](http://kanaka.github.io/mal/process/cheatsheet.html) that | |
190 | summarizes the key changes at each step. | |
0f4ca9d1 | 191 | |
daa1cf3f JM |
192 | If you get completely stuck and are feeling like giving up, then you |
193 | should "cheat" by referring to the same step or functionality in | |
194 | a existing implementation language. You are here to learn, not to take | |
195 | a test, so do not feel bad about it. Okay, you should feel a little | |
196 | bit bad about it. | |
0f4ca9d1 JM |
197 | |
198 | ||
199 | ## The Make-A-Lisp Process | |
200 | ||
201 | In the steps that follow the name of the target language is "quux" and | |
202 | the file extension for that language is "qx". | |
203 | ||
204 | ||
205 | <a name="step0"></a> | |
206 | ||
207 | ### Step 0: The REPL | |
208 | ||
209 | ![step0_repl architecture](step0_repl.png) | |
210 | ||
211 | This step is basically just creating a skeleton of your interpreter. | |
212 | ||
213 | * Create a `step0_repl.qx` file in `quux/`. | |
214 | ||
215 | * Add the 4 trivial functions `READ`, `EVAL`, `PRINT`, and `rep` | |
216 | (read-eval-print). `READ`, `EVAL`, and `PRINT` are basically just | |
217 | stubs that return their first parameter (a string if your target | |
218 | language is a statically typed) and `rep` calls them in order | |
219 | passing the return to the input of the next. | |
220 | ||
58ba5af4 JM |
221 | * Add a main loop that repeatedly prints a prompt (needs to be |
222 | "user> " for later tests to pass), gets a line of input from the | |
223 | user, calls `rep` with that line of input, and then prints out the | |
224 | result from `rep`. It should also exit when you send it an EOF | |
225 | (often Ctrl-D). | |
0f4ca9d1 JM |
226 | |
227 | * If you are using a compiled (ahead-of-time rather than just-in-time) | |
228 | language, then create a Makefile (or appropriate project definition | |
229 | file) in your directory. | |
230 | ||
daa1cf3f JM |
231 | It is time to run your first tests. This will check that your program |
232 | does input and output in a way that can be captured by the test | |
233 | harness. Go to the top level and run the following: | |
234 | ``` | |
e5737b08 | 235 | make "test^quux^step0" |
daa1cf3f | 236 | ``` |
0f4ca9d1 JM |
237 | |
238 | Add and then commit your new `step0_repl.qx` and `Makefile` to git. | |
239 | ||
240 | Congratulations! You have just completed the first step of the | |
241 | make-a-lisp process. | |
242 | ||
243 | ||
bd62ff74 | 244 | #### Optional: |
0f4ca9d1 JM |
245 | |
246 | * Add full line editing and command history support to your | |
247 | interpreter REPL. Many languages have a library/module that provide | |
248 | line editing support. Another option if your language supports it is | |
249 | to use an FFI (foreign function interface) to load and call directly | |
94a954f5 | 250 | into GNU readline, editline, or linenoise library. Add line |
0f4ca9d1 JM |
251 | editing interface code to `readline.qx` |
252 | ||
253 | ||
254 | <a name="step1"></a> | |
255 | ||
256 | ### Step 1: Read and Print | |
257 | ||
258 | ![step1_read_print architecture](step1_read_print.png) | |
259 | ||
260 | In this step, your interpreter will "read" the string from the user | |
261 | and parse it into an internal tree data structure (an abstract syntax | |
262 | tree) and then take that data structure and "print" it back to | |
263 | a string. | |
264 | ||
265 | In non-lisp languages, this step (called "lexing and parsing") can be | |
266 | one of the most complicated parts of the compiler/interpreter. In | |
267 | Lisp, the data structure that you want in memory is basically | |
268 | represented directly in the code that the programmer writes | |
269 | (homoiconicity). | |
270 | ||
271 | For example, if the string is "(+ 2 (* 3 4))" then the read function | |
272 | will process this into a tree structure that looks like this: | |
273 | ``` | |
274 | List | |
275 | / | \ | |
276 | / | \ | |
277 | / | \ | |
278 | Sym:+ Int:2 List | |
279 | / | \ | |
280 | / | \ | |
281 | / | \ | |
282 | Sym:* Int:3 Int:4 | |
283 | ``` | |
284 | ||
285 | Each left paren and its matching right paren (lisp "sexpr") becomes | |
286 | a node in the tree and everything else becomes a leaf in the tree. | |
287 | ||
288 | If you can find code for an implementation of a JSON encoder/decoder | |
289 | in your target language then you can probably just borrow and modify | |
290 | that and be 75% of the way done with this step. | |
291 | ||
292 | The rest of this section is going to assume that you are not starting | |
293 | from an existing JSON encoder/decoder, but that you do have access to | |
294 | a Perl compatible regular expressions (PCRE) module/library. You can | |
295 | certainly implement the reader using simple string operations, but it | |
296 | is more involved. The `make`, `ps` (postscript) and Haskell | |
297 | implementations have examples of a reader/parser without using regular | |
298 | expression support. | |
299 | ||
300 | * Copy `step0_repl.qx` to `step1_read_print.qx`. | |
301 | ||
302 | * Add a `reader.qx` file to hold functions related to the reader. | |
303 | ||
304 | * If the target language has objects types (OOP), then the next step | |
305 | is to create a simple stateful Reader object in `reader.qx`. This | |
306 | object will store the tokens and a position. The Reader object will | |
d717d991 | 307 | have two methods: `next` and `peek`. `next` returns the token at |
0f4ca9d1 JM |
308 | the current position and increments the position. `peek` just |
309 | returns the token at the current position. | |
310 | ||
311 | * Add a function `read_str` in `reader.qx`. This function | |
312 | will call `tokenizer` and then create a new Reader object instance | |
313 | with the tokens. Then it will call `read_form` with the Reader | |
314 | instance. | |
315 | ||
316 | * Add a function `tokenizer` in `reader.qx`. This function will take | |
2315fd53 | 317 | a single string and return an array/list |
0f4ca9d1 JM |
318 | of all the tokens (strings) in it. The following regular expression |
319 | (PCRE) will match all mal tokens. | |
320 | ``` | |
321 | [\s,]*(~@|[\[\]{}()'`~^@]|"(?:\\.|[^\\"])*"|;.*|[^\s\[\]{}('"`,;)]*) | |
322 | ``` | |
cfdf00cc OR |
323 | * For each match captured within the parenthesis starting at char 6 of the |
324 | regular expression a new token will be created. | |
325 | ||
326 | * `[\s,]*`: Matches any number of whitespaces or commas. This is not captured | |
327 | so it will be ignored and not tokenized. | |
328 | ||
329 | * `~@`: Captures the special two-characters `~@` (tokenized). | |
330 | ||
331 | * ```[\[\]{}()'`~^@]```: Captures any special single character, one of | |
185052f1 | 332 | ```[]{}()'`~^@``` (tokenized). |
cfdf00cc OR |
333 | |
334 | * `"(?:\\.|[^\\"])*"`: Starts capturing at a double-quote and stops at the | |
335 | next double-quote unless it was proceeded by a backslash in which case it | |
a85b8454 | 336 | includes it until the next double-quote (tokenized). |
cfdf00cc OR |
337 | |
338 | * `;.*`: Captures any sequence of characters starting with `;` (tokenized). | |
339 | ||
340 | * ```[^\s\[\]{}('"`,;)]*```: Captures a sequence of zero or more non special | |
341 | characters (e.g. symbols, numbers, "true", "false", and "nil") and is sort | |
342 | of the inverse of the one above that captures special characters (tokenized). | |
0f4ca9d1 JM |
343 | |
344 | * Add the function `read_form` to `reader.qx`. This function | |
345 | will peek at the first token in the Reader object and switch on the | |
346 | first character of that token. If the character is a left paren then | |
347 | `read_list` is called with the Reader object. Otherwise, `read_atom` | |
348 | is called with the Reader Object. The return value from `read_form` | |
349 | is a mal data type. If your target language is statically typed then | |
350 | you will need some way for `read_form` to return a variant or | |
351 | subclass type. For example, if your language is object oriented, | |
2315fd53 | 352 | then you can define a top level MalType (in `types.qx`) that all |
0f4ca9d1 JM |
353 | your mal data types inherit from. The MalList type (which also |
354 | inherits from MalType) will contains a list/array of other MalTypes. | |
355 | If your language is dynamically typed then you can likely just | |
356 | return a plain list/array of other mal types. | |
357 | ||
358 | * Add the function `read_list` to `reader.qx`. This function will | |
359 | repeatedly call `read_form` with the Reader object until it | |
360 | encounters a ')' token (if it reach EOF before reading a ')' then | |
361 | that is an error). It accumulates the results into a List type. If | |
362 | your language does not have a sequential data type that can hold mal | |
363 | type values you may need to implement one (in `types.qx`). Note | |
364 | that `read_list` repeatedly calls `read_form` rather than | |
a85b8454 | 365 | `read_atom`. This mutually recursive definition between `read_list` |
0f4ca9d1 JM |
366 | and `read_form` is what allows lists to contain lists. |
367 | ||
368 | * Add the function `read_atom` to `reader.qx`. This function will | |
369 | look at the contents of the token and return the appropriate scalar | |
370 | (simple/single) data type value. Initially, you can just implement | |
b84a50cc | 371 | numbers (integers) and symbols. This will allow you to proceed |
0f4ca9d1 JM |
372 | through the next couple of steps before you will need to implement |
373 | the other fundamental mal types: nil, true, false, and string. The | |
374 | remaining mal types: keyword, vector, hash-map, and atom do not | |
375 | need to be implemented until step 9 (but can be implemented at any | |
376 | point between this step and that). BTW, symbols types are just an | |
377 | object that contains a single string name value (some languages have | |
378 | symbol types already). | |
379 | ||
380 | * Add a file `printer.qx`. This file will contain a single function | |
381 | `pr_str` which does the opposite of `read_str`: take a mal data | |
382 | structure and return a string representation of it. But `pr_str` is | |
383 | much simpler and is basically just a switch statement on the type of | |
384 | the input object: | |
385 | ||
386 | * symbol: return the string name of the symbol | |
387 | * number: return the number as a string | |
388 | * list: iterate through each element of the list calling `pr_str` on | |
389 | it, then join the results with a space separator, and surround the | |
390 | final result with parens | |
391 | ||
392 | * Change the `READ` function in `step1_read_print.qx` to call | |
393 | `reader.read_str` and the `PRINT` function to call `printer.pr_str`. | |
394 | `EVAL` continues to simply return its input but the type is now | |
395 | a mal data type. | |
396 | ||
397 | You now have enough hooked up to begin testing your code. You can | |
398 | manually try some simple inputs: | |
399 | * `123` -> `123` | |
400 | * ` 123 ` -> `123` | |
401 | * `abc` -> `abc` | |
402 | * ` abc ` -> `abc` | |
403 | * `(123 456)` -> `(123 456)` | |
404 | * `( 123 456 789 ) ` -> `(123 456 789)` | |
6a767b0d | 405 | * `( + 2 (* 3 4) ) ` -> `(+ 2 (* 3 4))` |
0f4ca9d1 JM |
406 | |
407 | To verify that your code is doing more than just eliminating extra | |
408 | spaces (and not failing), you can instrument your `reader.qx` functions. | |
409 | ||
410 | Once you have gotten past those simple manual tests, it is time to run | |
411 | the full suite of step 1 tests. Go to the top level and run the | |
412 | following: | |
413 | ``` | |
e5737b08 | 414 | make "test^quux^step1" |
0f4ca9d1 JM |
415 | ``` |
416 | ||
417 | Fix any test failures related to symbols, numbers and lists. | |
418 | ||
419 | Depending on the functionality of your target language, it is likely | |
420 | that you have now just completed one of the most difficult steps. It | |
421 | is down hill from here. The remaining steps will probably be easier | |
422 | and each step will give progressively more bang for the buck. | |
423 | ||
45a8b3ca | 424 | #### Deferrable: |
0f4ca9d1 JM |
425 | |
426 | ||
0f4ca9d1 JM |
427 | * Add support for the other basic data type to your reader and printer |
428 | functions: string, nil, true, and false. These become mandatory at | |
8d78bc26 JM |
429 | step 4. When a string is read, the following transformations are |
430 | applied: a backslash followed by a doublequote is translated into | |
431 | a plain doublequote character, a backslash followed by "n" is | |
432 | translated into a newline, and a backslash followed by another | |
433 | backslash is translated into a single backslash. To properly print | |
434 | a string (for step 4 string functions), the `pr_str` function needs | |
435 | another parameter called `print_readably`. When `print_readably` is | |
436 | true, doublequotes, newlines, and backslashes are translated into | |
437 | their printed representations (the reverse of the reader). The | |
438 | `PRINT` function in the main program should call `pr_str` with | |
439 | print_readably set to true. | |
0f4ca9d1 | 440 | |
4b7ee8fa JM |
441 | * Add error checking to your reader functions to make sure parens |
442 | are properly matched. Catch and print these errors in your main | |
443 | loop. If your language does not have try/catch style bubble up | |
444 | exception handling, then you will need to add explicit error | |
445 | handling to your code to catch and pass on errors without crashing. | |
446 | ||
447 | * Add support for reader macros which are forms that are | |
448 | transformed into other forms during the read phase. Refer to | |
449 | `tests/step1_read_print.mal` for the form that these macros should | |
450 | take (they are just simple transformations of the token stream). | |
451 | ||
1c2eab90 | 452 | * Add support for the other mal types: keyword, vector, hash-map. |
2345a3da JM |
453 | * keyword: a keyword is a token that begins with a colon. A keyword |
454 | can just be stored as a string with special unicode prefix like | |
455 | 0x29E (or char 0xff/127 if the target language does not have good | |
456 | unicode support) and the printer translates strings with that | |
457 | prefix back to the keyword representation. This makes it easy to | |
458 | use keywords as hash map keys in most languages. You can also | |
459 | store keywords as a unique data type, but you will need to make | |
460 | sure they can be used as hash map keys (which may involve doing | |
461 | a similar prefixed translation anyways). | |
462 | * vector: a vector can be implemented with same underlying | |
463 | type as a list as long as there is some mechanism to keep track of | |
464 | the difference. You can use the same reader function for both | |
465 | lists and vectors by adding parameters for the starting and ending | |
466 | tokens. | |
467 | * hash-map: a hash-map is an associative data structure that maps | |
468 | strings to other mal values. If you implement keywords as prefixed | |
469 | strings, then you only need a native associative data structure | |
470 | which supports string keys. Clojure allows any value to be a hash | |
471 | map key, but the base functionality in mal is to support strings | |
472 | and keyword keys. Because of the representation of hash-maps as | |
473 | an alternating sequence of keys and values, you can probably use | |
474 | the same reader function for hash-maps as lists and vectors with | |
475 | parameters to indicate the starting and ending tokens. The odd | |
476 | tokens are then used for keys with the corresponding even tokens | |
477 | as the values. | |
478 | ||
0f4ca9d1 JM |
479 | * Add comment support to your reader. The tokenizer should ignore |
480 | tokens that start with ";". Your `read_str` function will need to | |
481 | properly handle when the tokenizer returns no values. The simplest | |
482 | way to do this is to return `nil` mal value. A cleaner option (that | |
483 | does not print `nil` at the prompt is to throw a special exception | |
484 | that causes the main loop to simply continue at the beginning of the | |
485 | loop without calling `rep`. | |
486 | ||
487 | ||
488 | <a name="step2"></a> | |
489 | ||
490 | ### Step 2: Eval | |
491 | ||
492 | ![step2_eval architecture](step2_eval.png) | |
493 | ||
494 | In step 1 your mal interpreter was basically just a way to validate | |
495 | input and eliminate extraneous white space. In this step you will turn | |
496 | your interpreter into a simple number calculator by adding | |
497 | functionality to the evaluator (`EVAL`). | |
498 | ||
499 | Compare the pseudocode for step 1 and step 2 to get a basic idea of | |
500 | the changes that will be made during this step: | |
501 | ``` | |
502 | diff -urp ../process/step1_read_print.txt ../process/step2_eval.txt | |
503 | ``` | |
504 | ||
505 | * Copy `step1_read_print.qx` to `step2_eval.qx`. | |
506 | ||
507 | * Define a simple initial REPL environment. This environment is an | |
508 | associative structure that maps symbols (or symbol names) to | |
509 | numeric functions. For example, in python this would look something | |
510 | like this: | |
511 | ``` | |
512 | repl_env = {'+': lambda a,b: a+b, | |
513 | '-': lambda a,b: a-b, | |
514 | '*': lambda a,b: a*b, | |
515 | '/': lambda a,b: int(a/b)} | |
516 | ``` | |
517 | ||
518 | * Modify the `rep` function to pass the REPL environment as the second | |
519 | parameter for the `EVAL` call. | |
520 | ||
521 | * Create a new function `eval_ast` which takes `ast` (mal data type) | |
522 | and an associative structure (the environment from above). | |
523 | `eval_ast` switches on the type of `ast` as follows: | |
524 | ||
525 | * symbol: lookup the symbol in the environment structure and return | |
0198b7a2 | 526 | the value or raise an error if no value is found |
0f4ca9d1 JM |
527 | * list: return a new list that is the result of calling `EVAL` on |
528 | each of the members of the list | |
529 | * otherwise just return the original `ast` value | |
530 | ||
531 | * Modify `EVAL` to check if the first parameter `ast` is a list. | |
532 | * `ast` is not a list: then return the result of calling `eval_ast` | |
533 | on it. | |
efa2daef | 534 | * `ast` is a empty list: return ast unchanged. |
0f4ca9d1 JM |
535 | * `ast` is a list: call `eval_ast` to get a new evaluated list. Take |
536 | the first item of the evaluated list and call it as function using | |
537 | the rest of the evaluated list as its arguments. | |
538 | ||
539 | If your target language does not have full variable length argument | |
540 | support (e.g. variadic, vararg, splats, apply) then you will need to | |
541 | pass the full list of arguments as a single parameter and split apart | |
542 | the individual values inside of every mal function. This is annoying, | |
543 | but workable. | |
544 | ||
545 | The process of taking a list and invoking or executing it to return | |
546 | something new is known in Lisp as the "apply" phase. | |
547 | ||
548 | Try some simple expressions: | |
549 | ||
550 | * `(+ 2 3)` -> `5` | |
551 | * `(+ 2 (* 3 4))` -> `14` | |
552 | ||
553 | The most likely challenge you will encounter is how to properly call | |
554 | a function references using an arguments list. | |
555 | ||
556 | Now go to the top level, run the step 2 tests and fix the errors. | |
557 | ``` | |
e5737b08 | 558 | make "test^quux^step2" |
0f4ca9d1 JM |
559 | ``` |
560 | ||
561 | You now have a simple prefix notation calculator! | |
562 | ||
0748211b DM |
563 | #### Deferrable: |
564 | ||
565 | * `eval_ast` should evaluate elements of vectors and hash-maps. Add the | |
566 | following cases in `eval_ast`: | |
567 | * If `ast` is a vector: return a new vector that is the result of calling | |
568 | `EVAL` on each of the members of the vector. | |
569 | * If `ast` is a hash-map: return a new hash-map which consists of key-value | |
570 | pairs where the key is a key from the hash-map and the value is the result | |
571 | of calling `EVAL` on the corresponding value. | |
572 | ||
0f4ca9d1 JM |
573 | |
574 | <a name="step3"></a> | |
575 | ||
576 | ### Step 3: Environments | |
577 | ||
578 | ![step3_env architecture](step3_env.png) | |
579 | ||
580 | In step 2 you were already introduced to REPL environment (`repl_env`) | |
581 | where the basic numeric functions were stored and looked up. In this | |
582 | step you will add the ability to create new environments (`let*`) and | |
daa1cf3f | 583 | modify existing environments (`def!`). |
0f4ca9d1 JM |
584 | |
585 | A Lisp environment is an associative data structure that maps symbols (the | |
586 | keys) to values. But Lisp environments have an additional important | |
587 | function: they can refer to another environment (the outer | |
588 | environment). During environment lookups, if the current environment | |
589 | does not have the symbol, the lookup continues in the outer | |
590 | environment, and continues this way until the symbol is either found, | |
591 | or the outer environment is `nil` (the outermost environment in the | |
592 | chain). | |
593 | ||
594 | Compare the pseudocode for step 2 and step 3 to get a basic idea of | |
595 | the changes that will be made during this step: | |
596 | ``` | |
597 | diff -urp ../process/step2_eval.txt ../process/step3_env.txt | |
598 | ``` | |
599 | ||
d9c020b0 | 600 | * Copy `step2_eval.qx` to `step3_env.qx`. |
0f4ca9d1 JM |
601 | |
602 | * Create `env.qx` to hold the environment definition. | |
603 | ||
604 | * Define an `Env` object that is instantiated with a single `outer` | |
605 | parameter and starts with an empty associative data structure | |
606 | property `data`. | |
607 | ||
608 | * Define three methods for the Env object: | |
609 | * set: takes a symbol key and a mal value and adds to the `data` | |
610 | structure | |
611 | * find: takes a symbol key and if the current environment contains | |
612 | that key then return the environment. If no key is found and outer | |
613 | is not `nil` then call find (recurse) on the outer environment. | |
614 | * get: takes a symbol key and uses the `find` method to locate the | |
615 | environment with the key, then returns the matching value. If no | |
616 | key is found up the outer chain, then throws/raises a "not found" | |
617 | error. | |
618 | ||
169ddeb2 | 619 | * Update `step3_env.qx` to use the new `Env` type to create the |
0f4ca9d1 JM |
620 | repl_env (with a `nil` outer value) and use the `set` method to add |
621 | the numeric functions. | |
622 | ||
623 | * Modify `eval_ast` to call the `get` method on the `env` parameter. | |
624 | ||
625 | * Modify the apply section of `EVAL` to switch on the first element of | |
626 | the list: | |
627 | * symbol "def!": call the set method of the current environment | |
628 | (second parameter of `EVAL` called `env`) using the unevaluated | |
629 | first parameter (second list element) as the symbol key and the | |
630 | evaluated second parameter as the value. | |
a727d6e8 | 631 | * symbol "let\*": create a new environment using the current |
0f4ca9d1 | 632 | environment as the outer value and then use the first parameter as |
a727d6e8 MK |
633 | a list of new bindings in the "let\*" environment. Take the second |
634 | element of the binding list, call `EVAL` using the new "let\*" | |
0f4ca9d1 | 635 | environment as the evaluation environment, then call `set` on the |
a727d6e8 | 636 | "let\*" environment using the first binding list element as the key |
0f4ca9d1 JM |
637 | and the evaluated second element as the value. This is repeated |
638 | for each odd/even pair in the binding list. Note in particular, | |
639 | the bindings earlier in the list can be referred to by later | |
640 | bindings. Finally, the second parameter (third element) of the | |
a727d6e8 | 641 | original `let*` form is evaluated using the new "let\*" environment |
0f4ca9d1 JM |
642 | and the result is returned as the result of the `let*` (the new |
643 | let environment is discarded upon completion). | |
644 | * otherwise: call `eval_ast` on the list and apply the first element | |
645 | to the rest as before. | |
646 | ||
647 | `def!` and `let*` are Lisp "specials" (or "special atoms") which means | |
648 | that they are language level features and more specifically that the | |
649 | rest of the list elements (arguments) may be evaluated differently (or | |
650 | not at all) unlike the default apply case where all elements of the | |
651 | list are evaluated before the first element is invoked. Lists which | |
652 | contain a "special" as the first element are known as "special forms". | |
34935988 | 653 | They are special because they follow special evaluation rules. |
0f4ca9d1 JM |
654 | |
655 | Try some simple environment tests: | |
656 | ||
657 | * `(def! a 6)` -> `6` | |
658 | * `a` -> `6` | |
659 | * `(def! b (+ a 2))` -> `8` | |
660 | * `(+ a b)` -> `14` | |
661 | * `(let* (c 2) c)` -> `2` | |
662 | ||
663 | Now go to the top level, run the step 3 tests and fix the errors. | |
664 | ``` | |
e5737b08 | 665 | make "test^quux^step3" |
0f4ca9d1 JM |
666 | ``` |
667 | ||
668 | You mal implementation is still basically just a numeric calculator | |
669 | with save/restore capability. But you have set the foundation for step | |
670 | 4 where it will begin to feel like a real programming language. | |
671 | ||
672 | ||
673 | An aside on mutation and typing: | |
674 | ||
675 | The "!" suffix on symbols is used to indicate that this symbol refers | |
676 | to a function that mutates something else. In this case, the `def!` | |
677 | symbol indicates a special form that will mutate the current | |
678 | environment. Many (maybe even most) of runtime problems that are | |
679 | encountered in software engineering are a result of mutation. By | |
680 | clearly marking code where mutation may occur, you can more easily | |
681 | track down the likely cause of runtime problems when they do occur. | |
682 | ||
683 | Another cause of runtime errors is type errors, where a value of one | |
684 | type is unexpectedly treated by the program as a different and | |
685 | incompatible type. Statically typed languages try to make the | |
686 | programmer solve all type problems before the program is allowed to | |
687 | run. Most Lisp variants tend to be dynamically typed (types of values | |
688 | are checked when they are actually used at runtime). | |
689 | ||
690 | As an aside-aside: The great debate between static and dynamic typing | |
4e3fd73b MK |
691 | can be understood by following the money. Advocates of strict static |
692 | typing use words like "correctness" and "safety" and thus get | |
0f4ca9d1 JM |
693 | government and academic funding. Advocates of dynamic typing use words |
694 | like "agile" and "time-to-market" and thus get venture capital and | |
695 | commercial funding. | |
696 | ||
697 | ||
698 | <a name="step4"></a> | |
699 | ||
700 | ### Step 4: If Fn Do | |
701 | ||
702 | ![step4_if_fn_do architecture](step4_if_fn_do.png) | |
703 | ||
704 | In step 3 you added environments and the special forms for | |
705 | manipulating environments. In this step you will add 3 new special | |
706 | forms (`if`, `fn*` and `do`) and add several more core functions to | |
707 | the default REPL environment. Our new architecture will look like | |
708 | this: | |
709 | ||
710 | The `fn*` special form is how new user-defined functions are created. | |
711 | In some Lisps, this special form is named "lambda". | |
712 | ||
713 | Compare the pseudocode for step 3 and step 4 to get a basic idea of | |
714 | the changes that will be made during this step: | |
715 | ``` | |
716 | diff -urp ../process/step3_env.txt ../process/step4_if_fn_do.txt | |
717 | ``` | |
718 | ||
719 | * Copy `step3_env.qx` to `step4_if_fn_do.qx`. | |
720 | ||
721 | * If you have not implemented reader and printer support (and data | |
722 | types) for `nil`, `true` and `false`, you will need to do so for | |
723 | this step. | |
724 | ||
725 | * Update the constructor/initializer for environments to take two new | |
726 | arguments: `binds` and `exprs`. Bind (`set`) each element (symbol) | |
727 | of the binds list to the respective element of the `exprs` list. | |
728 | ||
729 | * Add support to `printer.qx` to print functions values. A string | |
730 | literal like "#<function>" is sufficient. | |
731 | ||
1af1f7f1 | 732 | * Add the following special forms to `EVAL`: |
0f4ca9d1 | 733 | |
1af1f7f1 | 734 | * `do`: Evaluate all the elements of the list using `eval_ast` |
f558a0c8 | 735 | and return the final evaluated element. |
0f4ca9d1 JM |
736 | * `if`: Evaluate the first parameter (second element). If the result |
737 | (condition) is anything other than `nil` or `false`, then evaluate | |
1af1f7f1 | 738 | the second parameter (third element of the list) and return the |
0f4ca9d1 JM |
739 | result. Otherwise, evaluate the third parameter (fourth element) |
740 | and return the result. If condition is false and there is no third | |
741 | parameter, then just return `nil`. | |
742 | * `fn*`: Return a new function closure. The body of that closure | |
743 | does the following: | |
744 | * Create a new environment using `env` (closed over from outer | |
745 | scope) as the `outer` parameter, the first parameter (second | |
746 | list element of `ast` from the outer scope) as the `binds` | |
747 | parameter, and the parameters to the closure as the `exprs` | |
748 | parameter. | |
749 | * Call `EVAL` on the second parameter (third list element of `ast` | |
750 | from outer scope), using the new environment. Use the result as | |
751 | the return value of the closure. | |
752 | ||
753 | If your target language does not support closures, then you will need | |
754 | to implement `fn*` using some sort of structure or object that stores | |
755 | the values being closed over: the first and second elements of the | |
756 | `ast` list (function parameter list and function body) and the current | |
757 | environment `env`. In this case, your native functions will need to be | |
758 | wrapped in the same way. You will probably also need a method/function | |
759 | that invokes your function object/structure for the default case of | |
760 | the apply section of `EVAL`. | |
761 | ||
762 | Try out the basic functionality you have implemented: | |
763 | ||
764 | * `(fn* [a] a)` -> `#<function>` | |
765 | * `( (fn* [a] a) 7)` -> `7` | |
766 | * `( (fn* [a] (+ a 1)) 10)` -> `11` | |
767 | * `( (fn* [a b] (+ a b)) 2 3)` -> `5` | |
768 | ||
769 | * Add a new file `core.qx` and define an associative data structure | |
770 | `ns` (namespace) that maps symbols to functions. Move the numeric | |
771 | function definitions into this structure. | |
772 | ||
773 | * Modify `step4_if_fn_do.qx` to iterate through the `core.ns` | |
774 | structure and add (`set`) each symbol/function mapping to the | |
775 | REPL environment (`repl_env`). | |
776 | ||
777 | * Add the following functions to `core.ns`: | |
4b7ee8fa JM |
778 | * `prn`: call `pr_str` on the first parameter with `print_readably` |
779 | set to true, prints the result to the screen and then return | |
a1eb30fc | 780 | `nil`. Note that the full version of `prn` is a deferrable below. |
0f4ca9d1 JM |
781 | * `list`: take the parameters and return them as a list. |
782 | * `list?`: return true if the first parameter is a list, false | |
783 | otherwise. | |
784 | * `empty?`: treat the first parameter as a list and return true if | |
785 | the list is empty and false if it contains any elements. | |
786 | * `count`: treat the first parameter as a list and return the number | |
787 | of elements that it contains. | |
788 | * `=`: compare the first two parameters and return true if they are | |
789 | the same type and contain the same value. In the case of equal | |
790 | length lists, each element of the list should be compared for | |
791 | equality and if they are the same return true, otherwise false. | |
792 | * `<`, `<=`, `>`, and `>=`: treat the first two parameters as | |
793 | numbers and do the corresponding numeric comparison, returning | |
794 | either true or false. | |
795 | ||
796 | Now go to the top level, run the step 4 tests. There are a lot of | |
797 | tests in step 4 but all of the non-optional tests that do not involve | |
798 | strings should be able to pass now. | |
799 | ||
800 | ``` | |
e5737b08 | 801 | make "test^quux^step4" |
0f4ca9d1 JM |
802 | ``` |
803 | ||
804 | Your mal implementation is already beginning to look like a real | |
805 | language. You have flow control, conditionals, user-defined functions | |
806 | with lexical scope, side-effects (if you implement the string | |
1380d8c1 JM |
807 | functions), etc. However, our little interpreter has not quite reached |
808 | Lisp-ness yet. The next several steps will take your implementation | |
809 | from a neat toy to a full featured language. | |
0f4ca9d1 | 810 | |
45a8b3ca | 811 | #### Deferrable: |
0f4ca9d1 JM |
812 | |
813 | * Implement Clojure-style variadic function parameters. Modify the | |
814 | constructor/initializer for environments, so that if a "&" symbol is | |
815 | encountered in the `binds` list, the next symbol in the `binds` list | |
816 | after the "&" is bound to the rest of the `exprs` list that has not | |
817 | been bound yet. | |
818 | ||
1c2eab90 | 819 | * Define a `not` function using mal itself. In `step4_if_fn_do.qx` |
0f4ca9d1 JM |
820 | call the `rep` function with this string: |
821 | "(def! not (fn* (a) (if a false true)))". | |
822 | ||
823 | * Implement the strings functions in `core.qx`. To implement these | |
824 | functions, you will need to implement the string support in the | |
45a8b3ca | 825 | reader and printer (deferrable section of step 1). Each of the string |
0f4ca9d1 JM |
826 | functions takes multiple mal values, prints them (`pr_str`) and |
827 | joins them together into a new string. | |
828 | * `pr-str`: calls `pr_str` on each argument with `print_readably` | |
829 | set to true, joins the results with " " and returns the new | |
830 | string. | |
831 | * `str`: calls `pr_str` on each argument with `print_readably` set | |
832 | to false, concatenates the results together ("" separator), and | |
833 | returns the new string. | |
834 | * `prn`: calls `pr_str` on each argument with `print_readably` set | |
835 | to true, joins the results with " ", prints the string to the | |
836 | screen and then returns `nil`. | |
837 | * `println`: calls `pr_str` on each argument with `print_readably` set | |
838 | to false, joins the results with " ", prints the string to the | |
839 | screen and then returns `nil`. | |
840 | ||
841 | ||
842 | <a name="step5"></a> | |
843 | ||
844 | ### Step 5: Tail call optimization | |
845 | ||
846 | ![step5_tco architecture](step5_tco.png) | |
847 | ||
848 | In step 4 you added special forms `do`, `if` and `fn*` and you defined | |
849 | some core functions. In this step you will add a Lisp feature called | |
850 | tail call optimization (TCO). Also called "tail recursion" or | |
851 | sometimes just "tail calls". | |
852 | ||
853 | Several of the special forms that you have defined in `EVAL` end up | |
854 | calling back into `EVAL`. For those forms that call `EVAL` as the last | |
855 | thing that they do before returning (tail call) you will just loop back | |
856 | to the beginning of eval rather than calling it again. The advantage | |
857 | of this approach is that it avoids adding more frames to the call | |
1c2eab90 JK |
858 | stack. This is especially important in Lisp languages because they tend |
859 | to prefer using recursion instead of iteration for control structures. | |
860 | (Though some Lisps, such as Common Lisp, have iteration.) However, with | |
861 | tail call optimization, recursion can be made as stack efficient as | |
862 | iteration. | |
0f4ca9d1 JM |
863 | |
864 | Compare the pseudocode for step 4 and step 5 to get a basic idea of | |
865 | the changes that will be made during this step: | |
866 | ``` | |
867 | diff -urp ../process/step4_if_fn_do.txt ../process/step5_tco.txt | |
868 | ``` | |
869 | ||
0303d604 | 870 | * Copy `step4_if_fn_do.qx` to `step5_tco.qx`. |
0f4ca9d1 JM |
871 | |
872 | * Add a loop (e.g. while true) around all code in `EVAL`. | |
873 | ||
874 | * Modify each of the following form cases to add tail call recursion | |
875 | support: | |
876 | * `let*`: remove the final `EVAL` call on the second `ast` argument | |
877 | (third list element). Set `env` (i.e. the local variable passed in | |
878 | as second parameter of `EVAL`) to the new let environment. Set | |
879 | `ast` (i.e. the local variable passed in as first parameter of | |
880 | `EVAL`) to be the second `ast` argument. Continue at the beginning | |
881 | of the loop (no return). | |
882 | * `do`: change the `eval_ast` call to evaluate all the parameters | |
4e7296f9 JM |
883 | except for the last (2nd list element up to but not including |
884 | last). Set `ast` to the last element of `ast`. Continue | |
0f4ca9d1 JM |
885 | at the beginning of the loop (`env` stays unchanged). |
886 | * `if`: the condition continues to be evaluated, however, rather | |
887 | than evaluating the true or false branch, `ast` is set to the | |
888 | unevaluated value of the chosen branch. Continue at the beginning | |
889 | of the loop (`env` is unchanged). | |
890 | ||
891 | * The return value from the `fn*` special form will now become an | |
892 | object/structure with attributes that allow the default invoke case | |
893 | of `EVAL` to do TCO on mal functions. Those attributes are: | |
0f4ca9d1 JM |
894 | * `ast`: the second `ast` argument (third list element) representing |
895 | the body of the function. | |
896 | * `params`: the first `ast` argument (second list element) | |
897 | representing the parameter names of the function. | |
898 | * `env`: the current value of the `env` parameter of `EVAL`. | |
4e7296f9 JM |
899 | * `fn`: the original function value (i.e. what was return by `fn*` |
900 | in step 4). Note that this is deferrable until step 9 when it is | |
16ca60da JM |
901 | required for the `map` and `apply` core functions). You will also |
902 | need it in step 6 if you choose to not to defer atoms/`swap!` from | |
903 | that step. | |
0f4ca9d1 JM |
904 | |
905 | * The default "apply"/invoke case of `EVAL` must now be changed to | |
906 | account for the new object/structure returned by the `fn*` form. | |
907 | Continue to call `eval_ast` on `ast`. The first element is `f`. | |
908 | Switch on the type of `f`: | |
909 | * regular function (not one defined by `fn*`): apply/invoke it as | |
4e7296f9 | 910 | before (in step 4). |
0f4ca9d1 JM |
911 | * a `fn*` value: set `ast` to the `ast` attribute of `f`. Generate |
912 | a new environment using the `env` and `params` attributes of `f` | |
913 | as the `outer` and `binds` arguments and rest `ast` arguments | |
914 | (list elements 2 through the end) as the `exprs` argument. Set | |
915 | `env` to the new environment. Continue at the beginning of the loop. | |
916 | ||
917 | Run some manual tests from previous steps to make sure you have not | |
918 | broken anything by adding TCO. | |
919 | ||
920 | Now go to the top level, run the step 5 tests. | |
921 | ||
922 | ``` | |
e5737b08 | 923 | make "test^quux^step5" |
0f4ca9d1 JM |
924 | ``` |
925 | ||
926 | Look at the step 5 test file `tests/step5_tco.mal`. The `sum-to` | |
927 | function cannot be tail call optimized because it does something after | |
928 | the recursive call (`sum-to` calls itself and then does the addition). | |
929 | Lispers say that the `sum-to` is not in tail position. The `sum2` | |
930 | function however, calls itself from tail position. In other words, the | |
931 | recursive call to `sum2` is the last action that `sum2` does. Calling | |
932 | `sum-to` with a large value will cause a stack overflow exception in | |
933 | most target languages (some have super-special tricks they use to | |
934 | avoid stack overflows). | |
935 | ||
936 | Congratulations, your mal implementation already has a feature (TCO) | |
937 | that most mainstream languages lack. | |
938 | ||
939 | ||
940 | <a name="step6"></a> | |
941 | ||
396d869e | 942 | ### Step 6: Files, Mutation, and Evil |
0f4ca9d1 JM |
943 | |
944 | ![step6_file architecture](step6_file.png) | |
945 | ||
946 | In step 5 you added tail call optimization. In this step you will add | |
947 | some string and file operations and give your implementation a touch | |
948 | of evil ... er, eval. And as long as your language supports function | |
949 | closures, this step will be quite simple. However, to complete this | |
950 | step, you must implement string type support, so if you have been | |
951 | holding off on that you will need to go back and do so. | |
952 | ||
953 | Compare the pseudocode for step 5 and step 6 to get a basic idea of | |
954 | the changes that will be made during this step: | |
955 | ``` | |
956 | diff -urp ../process/step5_tco.txt ../process/step6_file.txt | |
957 | ``` | |
958 | ||
ffd31966 JM |
959 | * Copy `step5_tco.qx` to `step6_file.qx`. |
960 | ||
0f4ca9d1 JM |
961 | * Add two new string functions to the core namespaces: |
962 | * `read-string`: this function just exposes the `read_str` function | |
963 | from the reader. If your mal string type is not the same as your | |
964 | target language (e.g. statically typed language) then your | |
965 | `read-string` function will need to unbox (extract) the raw string | |
966 | from the mal string type in order to call `read_str`. | |
967 | * `slurp`: this function takes a file name (string) and returns the | |
968 | contents of the file as a string. Once again, if your mal string | |
969 | type wraps a raw target language string, then you will need to | |
970 | unmarshall (extract) the string parameter to get the raw file name | |
971 | string and marshall (wrap) the result back to a mal string type. | |
972 | ||
6fef8e58 JM |
973 | * In your main program, add a new symbol "eval" to your REPL |
974 | environment. The value of this new entry is a function that takes | |
975 | a single argument `ast`. The closure calls the your `EVAL` function | |
976 | using the `ast` as the first argument and the REPL environment | |
977 | (closed over from outside) as the second argument. The result of | |
978 | the `EVAL` call is returned. This simple but powerful addition | |
979 | allows your program to treat mal data as a mal program. For example, | |
cfdf00cc | 980 | you can now to this: |
6fef8e58 JM |
981 | ``` |
982 | (def! mal-prog (list + 1 2)) | |
983 | (eval mal-prog) | |
984 | ``` | |
0f4ca9d1 JM |
985 | |
986 | * Define a `load-file` function using mal itself. In your main | |
987 | program call the `rep` function with this string: | |
988 | "(def! load-file (fn* (f) (eval (read-string (str \"(do \" (slurp f) \")\")))))". | |
989 | ||
990 | Try out `load-file`: | |
991 | * `(load-file "../tests/incA.mal")` -> `9` | |
992 | * `(inc4 3)` -> `7` | |
993 | ||
994 | The `load-file` function does the following: | |
995 | * Call `slurp` to read in a file by name. Surround the contents with | |
996 | "(do ...)" so that the whole file will be treated as a single | |
997 | program AST (abstract syntax tree). | |
998 | * Call `read-string` on the string returned from `slurp`. This uses | |
999 | the reader to read/convert the file contents into mal data/AST. | |
1000 | * Call `eval` (the one in the REPL environment) on the AST returned | |
1001 | from `read-string` to "run" it. | |
1002 | ||
627bd6f7 DM |
1003 | Besides adding file and eval support, we'll add support for the atom data type |
1004 | in this step. An atom is the Mal way to represent *state*; it is | |
1005 | heavily inspired by [Clojure's atoms](http://clojure.org/state). An atom holds | |
1006 | a reference to a single Mal value of any type; it supports reading that Mal value | |
1007 | and *modifying* the reference to point to another Mal value. Note that this is | |
1008 | the only Mal data type that is mutable (but the Mal values it refers to are | |
1009 | still immutable; immutability is explained in greater detail in step 7). | |
a85b8454 | 1010 | You'll need to add 5 functions to the core namespace to support atoms: |
627bd6f7 DM |
1011 | |
1012 | * `atom`: Takes a Mal value and returns a new atom which points to that Mal value. | |
1013 | * `atom?`: Takes an argument and returns `true` if the argument is an atom. | |
1014 | * `deref`: Takes an atom argument and returns the Mal value referenced by this atom. | |
1015 | * `reset!`: Takes an atom and a Mal value; the atom is modified to refer to | |
1016 | the given Mal value. The Mal value is returned. | |
1017 | * `swap!`: Takes an atom, a function, and zero or more function arguments. The | |
1c2eab90 | 1018 | atom's value is modified to the result of applying the function with the atom's |
627bd6f7 DM |
1019 | value as the first argument and the optionally given function arguments as |
1020 | the rest of the arguments. The new atom's value is returned. (Side note: Mal is | |
1021 | single-threaded, but in concurrent languages like Clojure, `swap!` promises | |
1022 | atomic update: `(swap! myatom (fn* [x] (+ 1 x)))` will always increase the | |
1023 | `myatom` counter by one and will not suffer from missing updates when the | |
1024 | atom is updated from multiple threads.) | |
1025 | ||
1026 | Optionally, you can add a reader macro `@` which will serve as a short form for | |
1027 | `deref`, so that `@a` is equivalent to `(deref a)`. In order to do that, modify | |
1028 | the conditional in reader `read_form` function and add a case which deals with | |
1029 | the `@` token: if the token is `@` (at sign) then return a new list that | |
1030 | contains the symbol `deref` and the result of reading the next form | |
1031 | (`read_form`). | |
1032 | ||
0f4ca9d1 | 1033 | Now go to the top level, run the step 6 tests. The optional tests will |
627bd6f7 DM |
1034 | need support from the reader for comments, vectors, hash-maps and the `@` |
1035 | reader macro: | |
0f4ca9d1 | 1036 | ``` |
e5737b08 | 1037 | make "test^quux^step6" |
0f4ca9d1 JM |
1038 | ``` |
1039 | ||
1040 | Congratulations, you now have a full-fledged scripting language that | |
6fef8e58 JM |
1041 | can run other mal programs. The `slurp` function loads a file as |
1042 | a string, the `read-string` function calls the mal reader to turn that | |
10a76646 | 1043 | string into data, and the `eval` function takes data and evaluates it |
6fef8e58 JM |
1044 | as a normal mal program. However, it is important to note that the |
1045 | `eval` function is not just for running external programs. Because mal | |
1046 | programs are regular mal data structures, you can dynamically generate | |
1047 | or manipulate those data structures before calling `eval` on them. | |
a85b8454 | 1048 | This isomorphism (same shape) between data and programs is known as |
6fef8e58 JM |
1049 | "homoiconicity". Lisp languages are homoiconic and this property |
1050 | distinguishes them from most other programming languages. | |
1051 | ||
1052 | You mal implementation is quite powerful already but the set of | |
1053 | functions that are available (from `core.qx`) is fairly limited. The | |
1054 | bulk of the functions you will add are described in step 9 and step A, | |
1055 | but you will begin to flesh them out over the next few steps to | |
1056 | support quoting (step 7) and macros (step 8). | |
0f4ca9d1 JM |
1057 | |
1058 | ||
45a8b3ca | 1059 | #### Deferrable: |
0f4ca9d1 JM |
1060 | |
1061 | * Add the ability to run another mal program from the command line. | |
1062 | Prior to the REPL loop, check if your mal implementation is called | |
1063 | with command line arguments. If so, treat the first argument as | |
1064 | a filename and use `rep` to call `load-file` on that filename, and | |
1065 | finally exit/terminate execution. | |
1066 | ||
1067 | * Add the rest of the command line arguments to your REPL environment | |
1068 | so that programs that are run with `load-file` have access to their | |
a85b8454 | 1069 | calling environment. Add a new "\*ARGV\*" (symbol) entry to your REPL |
0f4ca9d1 JM |
1070 | environment. The value of this entry should be the rest of the |
1071 | command line arguments as a mal list value. | |
1072 | ||
1073 | ||
1074 | <a name="step7"></a> | |
1075 | ||
1076 | ### Step 7: Quoting | |
1077 | ||
1078 | ![step7_quote architecture](step7_quote.png) | |
1079 | ||
1080 | In step 7 you will add the special forms `quote` and `quasiquote` and | |
1081 | add supporting core functions `cons` and `concat`. The two quote forms | |
1082 | add a powerful abstraction for manipulating mal code itself | |
1083 | (meta-programming). | |
1084 | ||
0f4ca9d1 JM |
1085 | The `quote` special form indicates to the evaluator (`EVAL`) that the |
1086 | parameter should not be evaluated (yet). At first glance, this might | |
10a76646 | 1087 | not seem particularly useful but an example of what this enables is the |
0f4ca9d1 JM |
1088 | ability for a mal program to refer to a symbol itself rather than the |
1089 | value that it evaluates to. Likewise with lists. For example, consider | |
1090 | the following: | |
1091 | ||
1092 | * `(prn abc)`: this will lookup the symbol `abc` in the current | |
1093 | evaluation environment and print it. This will result in error if | |
1094 | `abc` is not defined. | |
1095 | * `(prn (quote abc))`: this will print "abc" (prints the symbol | |
1096 | itself). This will work regardless of whether `abc` is defined in | |
1097 | the current environment. | |
1098 | * `(prn (1 2 3))`: this will result in an error because `1` is not | |
1099 | a function and cannot be applied to the arguments `(2 3)`. | |
1100 | * `(prn (quote (1 2 3)))`: this will print "(1 2 3)". | |
1101 | * `(def! l (quote (1 2 3)))`: list quoting allows us to define lists | |
1102 | directly in the code (list literal). Another way of doing this is | |
1103 | with the list function: `(def! l (list 1 2 3))`. | |
1104 | ||
1105 | The second special quoting form is `quasiquote`. This allows a quoted | |
1106 | list to have internal elements of the list that are temporarily | |
1107 | unquoted (normal evaluation). There are two special forms that only | |
1108 | mean something within a quasiquoted list: `unquote` and | |
1109 | `splice-unquote`. These are perhaps best explained with some examples: | |
1110 | ||
1111 | * `(def! lst (quote (2 3)))` -> `(2 3)` | |
1112 | * `(quasiquote (1 (unquote lst)))` -> `(1 (2 3))` | |
1113 | * `(quasiquote (1 (splice-unquote lst)))` -> `(1 2 3)` | |
1114 | ||
1115 | The `unquote` form turns evaluation back on for its argument and the | |
1116 | result of evaluation is put in place into the quasiquoted list. The | |
1117 | `splice-unquote` also turns evaluation back on for its argument, but | |
1118 | the evaluated value must be a list which is then "spliced" into the | |
1119 | quasiquoted list. The true power of the quasiquote form will be | |
1c2eab90 | 1120 | manifest when it is used together with macros (in the next step). |
0f4ca9d1 | 1121 | |
ffd31966 JM |
1122 | Compare the pseudocode for step 6 and step 7 to get a basic idea of |
1123 | the changes that will be made during this step: | |
1124 | ``` | |
1125 | diff -urp ../process/step6_file.txt ../process/step7_quote.txt | |
1126 | ``` | |
1127 | ||
1128 | * Copy `step6_file.qx` to `step7_quote.qx`. | |
1129 | ||
0f4ca9d1 JM |
1130 | * Before implementing the quoting forms, you will need to implement |
1131 | * some supporting functions in the core namespace: | |
1132 | * `cons`: this function takes a list as its second | |
1133 | parameter and returns a new list that has the first argument | |
1134 | prepended to it. | |
1135 | * `concat`: this functions takes 0 or more lists as | |
1136 | parameters and returns a new list that is a concatenation of all | |
1137 | the list parameters. | |
1138 | ||
1139 | An aside on immutability: note that neither cons or concat mutate | |
1140 | their original list arguments. Any references to them (i.e. other | |
1141 | lists that they may be "contained" in) will still refer to the | |
1142 | original unchanged value. Mal, like Clojure, is a language which uses | |
1143 | immutable data structures. I encourage you to read about the power and | |
1144 | importance of immutability as implemented in Clojure (from which | |
1145 | Mal borrows most of its syntax and feature-set). | |
1146 | ||
1147 | * Add the `quote` special form. This form just returns its argument | |
1148 | (the second list element of `ast`). | |
1149 | ||
1150 | * Add the `quasiquote` special form. First implement a helper function | |
1151 | `is_pair` that returns true if the parameter is a non-empty list. | |
1152 | Then define a `quasiquote` function. This is called from `EVAL` with | |
1153 | the first `ast` argument (second list element) and then `ast` is set | |
1154 | to the result and execution continues at the top of the loop (TCO). | |
1155 | The `quasiquote` function takes a parameter `ast` and has the | |
1156 | following conditional: | |
1157 | 1. if `is_pair` of `ast` is false: return a new list containing: | |
1158 | a symbol named "quote" and `ast`. | |
1159 | 2. else if the first element of `ast` is a symbol named "unquote": | |
1160 | return the second element of `ast`. | |
1c2eab90 | 1161 | 3. if `is_pair` of the first element of `ast` is true and the first |
0f4ca9d1 JM |
1162 | element of first element of `ast` (`ast[0][0]`) is a symbol named |
1163 | "splice-unquote": return a new list containing: a symbol named | |
1164 | "concat", the second element of first element of `ast` | |
1165 | (`ast[0][1]`), and the result of calling `quasiquote` with the | |
1166 | second through last element of `ast`. | |
1167 | 4. otherwise: return a new list containing: a symbol named "cons", the | |
1168 | result of calling `quasiquote` on first element of `ast` | |
1c2eab90 | 1169 | (`ast[0]`), and the result of calling `quasiquote` with the second |
0f4ca9d1 JM |
1170 | through last element of `ast`. |
1171 | ||
1172 | ||
1173 | Now go to the top level, run the step 7 tests: | |
1174 | ``` | |
e5737b08 | 1175 | make "test^quux^step7" |
0f4ca9d1 JM |
1176 | ``` |
1177 | ||
1178 | Quoting is one of the more mundane functions available in mal, but do | |
1179 | not let that discourage you. Your mal implementation is almost | |
1180 | complete, and quoting sets the stage for the next very exiting step: | |
1181 | macros. | |
1182 | ||
1183 | ||
45a8b3ca | 1184 | #### Deferrable |
0f4ca9d1 JM |
1185 | |
1186 | * The full names for the quoting forms are fairly verbose. Most Lisp | |
1187 | languages have a short-hand syntax and Mal is no exception. These | |
1188 | short-hand syntaxes are known as reader macros because they allow us | |
1189 | to manipulate mal code during the reader phase. Macros that run | |
1190 | during the eval phase are just called "macros" and are described in | |
1191 | the next section. Expand the conditional with reader `read_form` | |
1192 | function to add the following four cases: | |
1193 | * token is "'" (single quote): return a new list that contains the | |
1194 | symbol "quote" and the result of reading the next form | |
1195 | (`read_form`). | |
8f41f75a | 1196 | * token is "\`" (back-tick): return a new list that contains the |
0f4ca9d1 JM |
1197 | symbol "quasiquote" and the result of reading the next form |
1198 | (`read_form`). | |
1199 | * token is "~" (tilde): return a new list that contains the | |
1200 | symbol "unquote" and the result of reading the next form | |
1201 | (`read_form`). | |
1202 | * token is "~@" (tilde + at sign): return a new list that contains | |
1203 | the symbol "splice-unquote" and the result of reading the next | |
1204 | form (`read_form`). | |
1205 | ||
1206 | * Add support for quoting of vectors. The `is_pair` function should | |
1207 | return true if the argument is a non-empty list or vector. `cons` | |
1208 | should also accept a vector as the second argument. The return value | |
1209 | is a list regardless. `concat` should support concatenation of | |
1210 | lists, vectors, or a mix or both. The result is always a list. | |
1211 | ||
1212 | ||
1213 | <a name="step8"></a> | |
1214 | ||
1215 | ### Step 8: Macros | |
1216 | ||
1217 | ![step8_macros architecture](step8_macros.png) | |
1218 | ||
a85b8454 | 1219 | Your mal implementation is now ready for one of the most lispy and |
0f4ca9d1 JM |
1220 | exciting of all programming concepts: macros. In the previous step, |
1221 | quoting enabled some simple manipulation data structures and therefore | |
1222 | manipulation of mal code (because the `eval` function from step | |
1223 | 6 turns mal data into code). In this step you will be able to mark mal | |
1224 | functions as macros which can manipulate mal code before it is | |
1225 | evaluated. In other words, macros are user-defined special forms. Or | |
1226 | to look at it another way, macros allow mal programs to redefine | |
1227 | the mal language itself. | |
1228 | ||
1229 | Compare the pseudocode for step 7 and step 8 to get a basic idea of | |
1230 | the changes that will be made during this step: | |
1231 | ``` | |
1232 | diff -urp ../process/step7_quote.txt ../process/step8_macros.txt | |
1233 | ``` | |
1234 | ||
ffd31966 JM |
1235 | * Copy `step7_quote.qx` to `step8_macros.qx`. |
1236 | ||
1237 | ||
0f4ca9d1 JM |
1238 | You might think that the infinite power of macros would require some |
1239 | sort of complex mechanism, but the implementation is actually fairly | |
1240 | simple. | |
1241 | ||
1242 | * Add a new attribute `is_macro` to mal function types. This should | |
1243 | default to false. | |
1244 | ||
1245 | * Add a new special form `defmacro!`. This is very similar to the | |
1246 | `def!` form, but before the evaluated value (mal function) is set in | |
1247 | the environment, the `is_macro` attribute should be set to true. | |
1248 | ||
1249 | * Add a `is_macro_call` function: This function takes arguments `ast` | |
1250 | and `env`. It returns true if `ast` is a list that contains a symbol | |
1251 | as the first element and that symbol refers to a function in the | |
1252 | `env` environment and that function has the `is_macro` attribute set | |
1253 | to true. Otherwise, it returns false. | |
1254 | ||
1255 | * Add a `macroexpand` function: This function takes arguments `ast` | |
1256 | and `env`. It calls `is_macro_call` with `ast` and `env` and loops | |
1257 | while that condition is true. Inside the loop, the first element of | |
1258 | the `ast` list (a symbol), is looked up in the environment to get | |
1259 | the macro function. This macro function is then called/applied with | |
1260 | the rest of the `ast` elements (2nd through the last) as arguments. | |
1261 | The return value of the macro call becomes the new value of `ast`. | |
1262 | When the loop completes because `ast` no longer represents a macro | |
1263 | call, the current value of `ast` is returned. | |
1264 | ||
1265 | * In the evaluator (`EVAL`) before the special forms switch (apply | |
1266 | section), perform macro expansion by calling the `macroexpand` | |
1267 | function with the current value of `ast` and `env`. Set `ast` to the | |
1268 | result of that call. If the new value of `ast` is no longer a list | |
9b4cfe03 JM |
1269 | after macro expansion, then return the result of calling `eval_ast` |
1270 | on it, otherwise continue with the rest of the apply section | |
1271 | (special forms switch). | |
0f4ca9d1 JM |
1272 | |
1273 | * Add a new special form condition for `macroexpand`. Call the | |
1274 | `macroexpand` function using the first `ast` argument (second list | |
1275 | element) and `env`. Return the result. This special form allows | |
1276 | a mal program to do explicit macro expansion without applying the | |
1277 | result (which can be useful for debugging macro expansion). | |
1278 | ||
1279 | Now go to the top level, run the step 8 tests: | |
1280 | ``` | |
e5737b08 | 1281 | make "test^quux^step8" |
0f4ca9d1 JM |
1282 | ``` |
1283 | ||
8a98ef9a JM |
1284 | There is a reasonably good chance that the macro tests will not pass |
1285 | the first time. Although the implementation of macros is fairly | |
1286 | simple, debugging runtime bugs with macros can be fairly tricky. If | |
1287 | you do run into subtle problems that are difficult to solve, let me | |
1288 | recommend a couple of approaches: | |
1289 | ||
1290 | * Use the macroexpand special form to eliminate one of the layers of | |
1291 | indirection (to expand but skip evaluate). This will often reveal | |
1292 | the source of the issue. | |
1293 | * Add a debug print statement to the top of your main `eval` function | |
1294 | (inside the TCO loop) to print the current value of `ast` (hint use | |
1295 | `pr_str` to get easier to debug output). Pull up the step8 | |
1296 | implementation from another language and uncomment its `eval` | |
1297 | function (yes, I give you permission to violate the rule this once). | |
1298 | Run the two side-by-side. The first difference is likely to point to | |
1299 | the bug. | |
1300 | ||
1301 | Congratulations! You now have a Lisp interpreter with a super power | |
1302 | that most non-Lisp languages can only dream of (I have it on good | |
1303 | authority that languages dream when you are not using them). If you | |
1304 | are not already familiar with Lisp macros, I suggest the following | |
a85b8454 | 1305 | exercise: write a recursive macro that handles postfixed mal code |
8a98ef9a JM |
1306 | (with the function as the last parameter instead of the first). Or |
1307 | not. I have not actually done so myself, but I have heard it is an | |
a85b8454 | 1308 | interesting exercise. |
8a98ef9a JM |
1309 | |
1310 | In the next step you will add try/catch style exception handling to | |
1311 | your implementation in addition to some new core functions. After | |
1312 | step9 you will be very close to having a fully self-hosting mal | |
1313 | implementation. Let us continue! | |
1314 | ||
0f4ca9d1 | 1315 | |
dc791440 | 1316 | #### Deferrable |
0f4ca9d1 JM |
1317 | |
1318 | * Add the following new core functions which are frequently used in | |
1319 | macro functions: | |
1320 | * `nth`: this function takes a list (or vector) and a number (index) | |
1321 | as arguments, returns the element of the list at the given index. | |
1322 | If the index is out of range, this function raises an exception. | |
1323 | * `first`: this function takes a list (or vector) as its argument | |
1324 | and return the first element. If the list (or vector) is empty or | |
1325 | is `nil` then `nil` is returned. | |
1326 | * `rest`: this function takes a list (or vector) as its argument and | |
1327 | returns a new list containing all the elements except the first. | |
1328 | ||
1329 | * In the main program, use the `rep` function to define two new | |
1330 | control structures macros. Here are the string arguments for `rep` | |
1331 | to define these macros: | |
1332 | * `cond`: "(defmacro! cond (fn* (& xs) (if (> (count xs) 0) (list 'if (first xs) (if (> (count xs) 1) (nth xs 1) (throw \"odd number of forms to cond\")) (cons 'cond (rest (rest xs)))))))" | |
dd4020b9 JM |
1333 | * Note that `cond` calls the `throw` function when `cond` is |
1334 | called with an odd number of args. The `throw` function is | |
1335 | implemented in the next step, but it will still serve it's | |
1336 | purpose here by causing an undefined symbol error. | |
0f4ca9d1 JM |
1337 | * `or`: "(defmacro! or (fn* (& xs) (if (empty? xs) nil (if (= 1 (count xs)) (first xs) `(let* (or_FIXME ~(first xs)) (if or_FIXME or_FIXME (or ~@(rest xs))))))))" |
1338 | ||
1339 | ||
ffd31966 JM |
1340 | <a name="step9"></a> |
1341 | ||
1342 | ### Step 9: Try | |
1343 | ||
1344 | ![step9_try architecture](step9_try.png) | |
1345 | ||
2345a3da JM |
1346 | In this step you will implement the final mal special form for |
1347 | error/exception handling: `try*/catch*`. You will also add several core | |
10a76646 AS |
1348 | functions to your implementation. In particular, you will enhance the |
1349 | functional programming pedigree of your implementation by adding the | |
2345a3da JM |
1350 | `apply` and `map` core functions. |
1351 | ||
ffd31966 | 1352 | Compare the pseudocode for step 8 and step 9 to get a basic idea of |
dbac60df | 1353 | the changes that will be made during this step: |
ffd31966 JM |
1354 | ``` |
1355 | diff -urp ../process/step8_macros.txt ../process/step9_try.txt | |
1356 | ``` | |
1357 | ||
1358 | * Copy `step8_macros.qx` to `step9_try.qx`. | |
1359 | ||
2345a3da JM |
1360 | * Add the `try*/catch*` special form to the EVAL function. The |
1361 | try catch form looks like this: `(try* A (catch* B C))`. The form | |
1362 | `A` is evaluated, if it throws an exception, then form `C` is | |
1c2eab90 | 1363 | evaluated with a new environment that binds the symbol `B` to the |
2345a3da JM |
1364 | value of the exception that was thrown. |
1365 | * If your target language has built-in try/catch style exception | |
1366 | handling then you are already 90% of the way done. Add a | |
1c2eab90 | 1367 | (native language) try/catch block that evaluates `A` within |
2345a3da JM |
1368 | the try block and catches all exceptions. If an exception is |
1369 | caught, then translate it to a mal type/value. For native | |
1370 | exceptions this is either the message string or a mal hash-map | |
1371 | that contains the message string and other attributes of the | |
1c2eab90 | 1372 | exception. When a regular mal type/value is used as an |
2345a3da JM |
1373 | exception, you will probably need to store it within a native |
1374 | exception type in order to be able to convey/transport it using | |
1375 | the native try/catch mechanism. Then you will extract the mal | |
1376 | type/value from the native exception. Create a new mal environment | |
1c2eab90 | 1377 | that binds `B` to the value of the exception. Finally, evaluate `C` |
2345a3da JM |
1378 | using that new environment. |
1379 | * If your target language does not have built-in try/catch style | |
1380 | exception handling then you have some extra work to do. One of the | |
1381 | most straightforward approaches is to create a a global error | |
1382 | variable that stores the thrown mal type/value. The complication | |
1383 | is that there are a bunch of places where you must check to see if | |
1384 | the global error state is set and return without proceeding. The | |
1385 | rule of thumb is that this check should happen at the top of your | |
1386 | EVAL function and also right after any call to EVAL (and after any | |
1387 | function call that might happen to call EVAL further down the | |
1388 | chain). Yes, it is ugly, but you were warned in the section on | |
1389 | picking a language. | |
1390 | ||
cfdf00cc | 1391 | * Add the `throw` core function. |
2345a3da JM |
1392 | * If your language supports try/catch style exception handling, then |
1393 | this function takes a mal type/value and throws/raises it as an | |
1394 | exception. In order to do this, you may need to create a custom | |
1395 | exception object that wraps a mal value/type. | |
1396 | * If your language does not support try/catch style exception | |
1397 | handling, then set the global error state to the mal type/value. | |
1398 | ||
1399 | * Add the `apply` and `map` core functions. In step 5, if you did not | |
1400 | add the original function (`fn`) to the structure returned from | |
bd62ff74 JM |
1401 | `fn*`, the you will need to do so now. |
1402 | * `apply`: takes at least two arguments. The first argument is | |
1403 | a function and the last argument is list (or vector). The | |
a85b8454 | 1404 | arguments between the function and the last argument (if there are |
bd62ff74 JM |
1405 | any) are concatenated with the final argument to create the |
1406 | arguments that are used to call the function. The apply | |
1407 | function allows a function to be called with arguments that are | |
1408 | contained in a list (or vector). In other words, `(apply F A B [C | |
1409 | D])` is equivalent to `(F A B C D)`. | |
1410 | * `map`: takes a function and a list (or vector) and evaluates the | |
1411 | function against every element of the list (or vector) one at | |
1412 | a time and returns the results as a list. | |
2345a3da JM |
1413 | |
1414 | * Add some type predicates core functions. In Lisp, predicates are | |
1415 | functions that return true/false (or true value/nil) and typically | |
bd62ff74 JM |
1416 | end in "?" or "p". |
1417 | * `nil?`: takes a single argument and returns true (mal true value) | |
1418 | if the argument is nil (mal nil value). | |
1419 | * `true?`: takes a single argument and returns true (mal true value) | |
1420 | if the argument is a true value (mal true value). | |
1421 | * `false?`: takes a single argument and returns true (mal true | |
1422 | value) if the argument is a false value (mal false value). | |
1423 | * `symbol?`: takes a single argument and returns true (mal true | |
1424 | value) if the argument is a symbol (mal symbol value). | |
1425 | ||
1426 | Now go to the top level, run the step 9 tests: | |
1427 | ``` | |
e5737b08 | 1428 | make "test^quux^step9" |
bd62ff74 JM |
1429 | ``` |
1430 | ||
1431 | Your mal implementation is now essentially a fully featured Lisp | |
1432 | interpreter. But if you stop now you will miss one of the most | |
1433 | satisfying and enlightening aspects of creating a mal implementation: | |
1434 | self-hosting. | |
2345a3da | 1435 | |
dc791440 | 1436 | #### Deferrable |
2345a3da | 1437 | |
e37d9b49 JM |
1438 | * Add the following new core functions: |
1439 | * `symbol`: takes a string and returns a new symbol with the string | |
1440 | as its name. | |
1441 | * `keyword`: takes a string and returns a keyword with the same name | |
1442 | (usually just be prepending the special keyword | |
1443 | unicode symbol). This function should also detect if the argument | |
1444 | is already a keyword and just return it. | |
1445 | * `keyword?`: takes a single argument and returns true (mal true | |
1446 | value) if the argument is a keyword, otherwise returns false (mal | |
1447 | false value). | |
1448 | * `vector`: takes a variable number of arguments and returns | |
1449 | a vector containing those arguments. | |
1450 | * `vector?`: takes a single argument and returns true (mal true | |
1451 | value) if the argument is a vector, otherwise returns false (mal | |
1452 | false value). | |
1453 | * `hash-map`: takes a variable but even number of arguments and | |
1454 | returns a new mal hash-map value with keys from the odd arguments | |
1455 | and values from the even arguments respectively. This is basically | |
1456 | the functional form of the `{}` reader literal syntax. | |
1457 | * `map?`: takes a single argument and returns true (mal true | |
1458 | value) if the argument is a hash-map, otherwise returns false (mal | |
1459 | false value). | |
1460 | * `assoc`: takes a hash-map as the first argument and the remaining | |
1461 | arguments are odd/even key/value pairs to "associate" (merge) into | |
1462 | the hash-map. Note that the original hash-map is unchanged | |
1463 | (remember, mal values are immutable), and a new hash-map | |
1464 | containing the old hash-maps key/values plus the merged key/value | |
1465 | arguments is returned. | |
1466 | * `dissoc`: takes a hash-map and a list of keys to remove from the | |
1467 | hash-map. Again, note that the original hash-map is unchanged and | |
1468 | a new hash-map with the keys removed is returned. Key arguments | |
1469 | that do not exist in the hash-map are ignored. | |
1470 | * `get`: takes a hash-map and a key and returns the value of looking | |
1471 | up that key in the hash-map. If the key is not found in the | |
1472 | hash-map then nil is returned. | |
1473 | * `contains?`: takes a hash-map and a key and returns true (mal true | |
1474 | value) if the key exists in the hash-map and false (mal false | |
1475 | value) otherwise. | |
1476 | * `keys`: takes a hash-map and returns a list (mal list value) of | |
1477 | all the keys in the hash-map. | |
1478 | * `vals`: takes a hash-map and returns a list (mal list value) of | |
1479 | all the values in the hash-map. | |
1480 | * `sequential?`: takes a single arguments and returns true (mal true | |
1481 | value) if it is a list or a vector, otherwise returns false (mal | |
1482 | false value). | |
ffd31966 JM |
1483 | |
1484 | ||
8569b2af | 1485 | <a name="stepA"></a> |
ffd31966 | 1486 | |
396d869e | 1487 | ### Step A: Metadata, Self-hosting and Interop |
ffd31966 | 1488 | |
90f618cb | 1489 | ![stepA_mal architecture](stepA_mal.png) |
ffd31966 | 1490 | |
bd62ff74 JM |
1491 | You have reached the final step of your mal implementation. This step |
1492 | is kind of a catchall for things that did not fit into other steps. | |
1493 | But most importantly, the changes you make in this step will unlock | |
1494 | the magical power known as "self-hosting". You might have noticed | |
1495 | that one of the languages that mal is implemented in is "mal". Any mal | |
1496 | implementation that is complete enough can run the mal implementation | |
1497 | of mal. You might need to pull out your hammock and ponder this for | |
1498 | a while if you have never built a compiler or interpreter before. Look | |
1499 | at the step source files for the mal implementation of mal (it is not | |
1500 | cheating now that you have reached step A). | |
1501 | ||
1502 | If you deferred the implementation of keywords, vectors and hash-maps, | |
1503 | now is the time to go back and implement them if you want your | |
1504 | implementation to self-host. | |
1505 | ||
ffd31966 JM |
1506 | Compare the pseudocode for step 9 and step A to get a basic idea of |
1507 | the changes that will be made during this step: | |
1508 | ``` | |
dbac60df | 1509 | diff -urp ../process/step9_try.txt ../process/stepA_mal.txt |
ffd31966 JM |
1510 | ``` |
1511 | ||
90f618cb | 1512 | * Copy `step9_try.qx` to `stepA_mal.qx`. |
ffd31966 | 1513 | |
396d869e JM |
1514 | * Add the `readline` core function. This functions takes a |
1515 | string that is used to prompt the user for input. The line of text | |
1516 | entered by the user is returned as a string. If the user sends an | |
1517 | end-of-file (usually Ctrl-D), then nil is returned. | |
1518 | ||
dd4020b9 JM |
1519 | * Add meta-data support to mal functions by adding a new metadata |
1520 | attribute on mal functions that refers to another mal value/type | |
1521 | (nil by default). Add the following metadata related core functions: | |
1522 | * `meta`: this takes a single mal function argument and returns the | |
1523 | value of the metadata attribute. | |
1524 | * `with-meta`: this function takes two arguments. The first argument | |
1525 | is a mal function and the second argument is another mal | |
1526 | value/type to set as metadata. A copy of the mal function is | |
1527 | returned that has its `meta` attribute set to the second argument. | |
1528 | Note that it is important that the environment and macro attribute | |
1529 | of mal function are retained when it is copied. | |
bd62ff74 | 1530 | |
33e37b68 JM |
1531 | * Add a new "\*host-language\*" (symbol) entry to your REPL |
1532 | environment. The value of this entry should be a mal string | |
0198b7a2 | 1533 | containing the name of the current implementation. |
33e37b68 JM |
1534 | |
1535 | * When the REPL starts up (as opposed to when it is called with | |
1536 | a script and/or arguments), call the `rep` function with this string | |
1537 | to print a startup header: | |
16ca60da | 1538 | "(println (str \"Mal [\" \*host-language\* \"]\"))". |
33e37b68 | 1539 | |
bd62ff74 | 1540 | |
8f41f75a | 1541 | Now go to the top level, run the step A tests: |
bd62ff74 | 1542 | ``` |
e5737b08 | 1543 | make "test^quux^stepA" |
bd62ff74 JM |
1544 | ``` |
1545 | ||
1546 | Once you have passed all the non-optional step A tests, it is time to | |
1547 | try self-hosting. Run your step A implementation as normal, but use | |
1548 | the file argument mode you added in step 6 to run a each of the step | |
1549 | from the mal implementation: | |
1550 | ``` | |
1551 | ./stepA_mal.qx ../mal/step1_read_print.mal | |
1552 | ./stepA_mal.qx ../mal/step2_eval.mal | |
1553 | ... | |
1554 | ./stepA_mal.qx ../mal/step9_try.mal | |
1555 | ./stepA_mal.qx ../mal/stepA_mal.mal | |
1556 | ``` | |
1557 | ||
10a76646 | 1558 | There is a very good chance that you will encounter an error at some |
bd62ff74 JM |
1559 | point while trying to run the mal in mal implementation steps above. |
1560 | Debugging failures that happen while self-hosting is MUCH more | |
1561 | difficult and mind bending. One of the best approaches I have | |
cfdf00cc | 1562 | personally found is to add prn statements to the mal implementation |
bd62ff74 JM |
1563 | step (not your own implementation of mal) that is causing problems. |
1564 | ||
1565 | Another approach I have frequently used is to pull out the code from | |
1566 | the mal implementation that is causing the problem and simplify it | |
1567 | step by step until you have a simple piece of mal code that still | |
1568 | reproduces the problem. Once the reproducer is simple enough you will | |
1569 | probably know where in your own implementation that problem is likely | |
1570 | to be. Please add your simple reproducer as a test case so that future | |
1571 | implementers will fix similar issues in their code before they get to | |
1572 | self-hosting when it is much more difficult to track down and fix. | |
1573 | ||
1574 | Once you can manually run all the self-hosted steps, it is time to run | |
1575 | all the tests in self-hosted mode: | |
1576 | ``` | |
e5737b08 | 1577 | make MAL_IMPL=quux "test^mal" |
bd62ff74 JM |
1578 | ``` |
1579 | ||
1580 | When you run into problems (which you almost certainly will), use the | |
1581 | same process described above to debug them. | |
1582 | ||
1583 | Congratulations!!! When all the tests pass, you should pause for | |
1584 | a moment and consider what you have accomplished. You have implemented | |
1585 | a Lisp interpreter that is powerful and complete enough to run a large | |
1586 | mal program which is itself an implementation of the mal language. You | |
1587 | might even be asking if you can continue the "inception" by using your | |
1588 | implementation to run a mal implementation which itself runs the mal | |
1589 | implementation. | |
1590 | ||
4881701a | 1591 | |
dc791440 | 1592 | #### Optional: gensym |
4881701a DM |
1593 | |
1594 | The `or` macro we introduced at step 8 has a bug. It defines a | |
1595 | variable called `or_FIXME`, which "shadows" such a binding from the | |
1596 | user's code (which uses the macro). If a user has a variable called | |
1597 | `or_FIXME`, it cannot be used as an `or` macro argument. In order to | |
1598 | fix that, we'll introduce `gensym`: a function which returns a symbol | |
1599 | which was never used before anywhere in the program. This is also an | |
1600 | example for the use of mal atoms to keep state (the state here being | |
1601 | the number of symbols produced by `gensym` so far). | |
1602 | ||
1603 | Previously you used `rep` to define the `or` macro. Remove that | |
1604 | definition and use `rep` to define the new counter, `gensym` function | |
1605 | and the clean `or` macro. Here are the string arguments you need to | |
1606 | pass to `rep`: | |
1607 | ``` | |
1608 | "(def! *gensym-counter* (atom 0))" | |
1609 | ||
1610 | "(def! gensym (fn* [] (symbol (str \"G__\" (swap! *gensym-counter* (fn* [x] (+ 1 x)))))))" | |
1611 | ||
1612 | "(defmacro! or (fn* (& xs) (if (empty? xs) nil (if (= 1 (count xs)) (first xs) (let* (condvar (gensym)) `(let* (~condvar ~(first xs)) (if ~condvar ~condvar (or ~@(rest xs)))))))))" | |
1613 | ``` | |
1614 | ||
1615 | For extra information read [Peter Seibel's thorough discussion about | |
1616 | `gensym` and leaking macros in Common Lisp](http://www.gigamonkeys.com/book/macros-defining-your-own.html#plugging-the-leaks). | |
1617 | ||
1618 | ||
dc791440 | 1619 | #### Optional additions |
bd62ff74 | 1620 | |
dd4020b9 JM |
1621 | * Add metadata support to other composite data types (lists, vectors |
1622 | and hash-maps), and to native functions. | |
50756494 DM |
1623 | * Add the following new core functions: |
1624 | * `time-ms`: takes no arguments and returns the number of | |
a85b8454 | 1625 | milliseconds since epoch (00:00:00 UTC January 1, 1970), or, if |
50756494 DM |
1626 | not possible, since another point in time (`time-ms` is usually |
1627 | used relatively to measure time durations). After `time-ms` is | |
dd4020b9 JM |
1628 | implemented, you can run the performance micro-benchmarks by |
1629 | running `make perf^quux`. | |
50756494 DM |
1630 | * `conj`: takes a collection and one or more elements as arguments |
1631 | and returns a new collection which includes the original | |
1632 | collection and the new elements. If the collection is a list, a | |
1633 | new list is returned with the elements inserted at the start of | |
1634 | the given list in opposite order; if the collection is a vector, a | |
1635 | new vector is returned with the elements added to the end of the | |
1636 | given vector. | |
396d869e JM |
1637 | * `string?`: returns true if the parameter is a string. |
1638 | * `seq`: takes a list, vector, string, or nil. If an empty list, | |
1639 | empty vector, or empty string ("") is passed in then nil is | |
1640 | returned. Otherwise, a list is returned unchanged, a vector is | |
1641 | converted into a list, and a string is converted to a list that | |
1642 | containing the original string split into single character | |
1643 | strings. | |
6440cc50 DM |
1644 | * For interop with the target language, add this core function: |
1645 | * `quux-eval`: takes a string, evaluates it in the target language, | |
1646 | and returns the result converted to the relevant Mal type. You | |
1647 | may also add other interop functions as you see fit; Clojure, for | |
1648 | example, has a function called `.` which allows calling Java | |
1649 | methods. If the target language is a static language, consider | |
1650 | using FFI or some language-specific reflection mechanism, if | |
1651 | available. The tests for `quux-eval` and any other interop | |
1652 | function should be added in `quux/tests/stepA_mal.mal` (see the | |
1653 | [tests for `lua-eval`](../lua/tests/stepA_mal.mal) as an example). | |
ffd31966 | 1654 | |
dd4020b9 JM |
1655 | ### Next Steps |
1656 | ||
1657 | * Join the #mal IRC channel. It's fairly quiet but there are bursts of | |
1658 | interesting conversation related to mal, Lisps, esoteric programming | |
1659 | languages, etc. | |
1660 | * If you have created an implementation for a new target language (or | |
1661 | a unique and interesting variant of an existing implementation), | |
1662 | consider sending a pull request to add it into the main mal | |
1663 | repository. The [FAQ](FAQ.md#add_implementation) describes general | |
1664 | requirements for getting an implementation merged into the main | |
1665 | repository. | |
1666 | * Take your interpreter implementation and have it emit source code in | |
1667 | the target language rather than immediately evaluating it. In other | |
1668 | words, create a compiler. | |
1669 | * Pick a new target language and implement mal in it. Pick a language | |
1670 | that is very different from any that you already know. | |
1671 | * Use your mal implementation to implement a real world project. Many | |
1672 | of these will force you to address interop. Some ideas: | |
1673 | * Web server (with mal as CGI language for extra points) | |
1674 | * An IRC/Slack chat bot | |
1675 | * An editor (GUI or curses) with mal as a scripting/extension | |
1676 | language. | |
1677 | * An AI player for a game like Chess or Go. | |
1678 | * Implement a feature in your mal implementation that is not covered | |
1679 | by this guide. Some ideas: | |
1680 | * Namespaces | |
1681 | * Multi-threading support | |
1682 | * Errors with line numbers and/or stack traces. | |
1683 | * Lazy sequences | |
1684 | * Clojure-style protocols | |
1685 | * Full call/cc (call-with-current-continuation) support | |
1686 | * Explicit TCO (i.e. `recur`) with tail-position error checking |