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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Guile Reference Manual. | |
40296bab | 3 | @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2006 |
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4 | @c Free Software Foundation, Inc. |
5 | @c See the file guile.texi for copying conditions. | |
6 | ||
7 | @page | |
8 | @node Read/Load/Eval | |
9 | @section Reading and Evaluating Scheme Code | |
10 | ||
11 | This chapter describes Guile functions that are concerned with reading, | |
12 | loading and evaluating Scheme code at run time. | |
13 | ||
14 | @menu | |
15 | * Scheme Syntax:: Standard and extended Scheme syntax. | |
16 | * Scheme Read:: Reading Scheme code. | |
17 | * Fly Evaluation:: Procedures for on the fly evaluation. | |
18 | * Loading:: Loading Scheme code from file. | |
19 | * Delayed Evaluation:: Postponing evaluation until it is needed. | |
20 | * Local Evaluation:: Evaluation in a local environment. | |
21 | * Evaluator Behaviour:: Modifying Guile's evaluator. | |
22 | @end menu | |
23 | ||
24 | ||
25 | @node Scheme Syntax | |
26 | @subsection Scheme Syntax: Standard and Guile Extensions | |
27 | ||
28 | @menu | |
29 | * Expression Syntax:: | |
30 | * Comments:: | |
31 | * Block Comments:: | |
32 | * Case Sensitivity:: | |
33 | * Keyword Syntax:: | |
34 | * Reader Extensions:: | |
35 | @end menu | |
36 | ||
37 | ||
38 | @node Expression Syntax | |
39 | @subsubsection Expression Syntax | |
40 | ||
41 | An expression to be evaluated takes one of the following forms. | |
42 | ||
43 | @table @nicode | |
44 | ||
45 | @item @var{symbol} | |
46 | A symbol is evaluated by dereferencing. A binding of that symbol is | |
47 | sought and the value there used. For example, | |
48 | ||
49 | @example | |
50 | (define x 123) | |
51 | x @result{} 123 | |
52 | @end example | |
53 | ||
54 | @item (@var{proc} @var{args}@dots{}) | |
55 | A parenthesised expression is a function call. @var{proc} and each | |
56 | argument are evaluated, then the function (which @var{proc} evaluated | |
57 | to) is called with those arguments. | |
58 | ||
59 | The order in which @var{proc} and the arguments are evaluated is | |
60 | unspecified, so be careful when using expressions with side effects. | |
61 | ||
62 | @example | |
63 | (max 1 2 3) @result{} 3 | |
64 | ||
65 | (define (get-some-proc) min) | |
66 | ((get-some-proc) 1 2 3) @result{} 1 | |
67 | @end example | |
68 | ||
69 | The same sort of parenthesised form is used for a macro invocation, | |
70 | but in that case the arguments are not evaluated. See the | |
71 | descriptions of macros for more on this (@pxref{Macros}, and | |
72 | @pxref{Syntax Rules}). | |
73 | ||
74 | @item @var{constant} | |
75 | Number, string, character and boolean constants evaluate ``to | |
76 | themselves'', so can appear as literals. | |
77 | ||
78 | @example | |
79 | 123 @result{} 123 | |
80 | 99.9 @result{} 99.9 | |
81 | "hello" @result{} "hello" | |
82 | #\z @result{} #\z | |
83 | #t @result{} #t | |
84 | @end example | |
85 | ||
86 | Note that an application must not attempt to modify literal strings, | |
87 | since they may be in read-only memory. | |
88 | ||
89 | @item (quote @var{data}) | |
90 | @itemx '@var{data} | |
91 | @findex quote | |
92 | @findex ' | |
93 | Quoting is used to obtain a literal symbol (instead of a variable | |
94 | reference), a literal list (instead of a function call), or a literal | |
95 | vector. @nicode{'} is simply a shorthand for a @code{quote} form. | |
96 | For example, | |
97 | ||
98 | @example | |
99 | 'x @result{} x | |
100 | '(1 2 3) @result{} (1 2 3) | |
101 | '#(1 (2 3) 4) @result{} #(1 (2 3) 4) | |
102 | (quote x) @result{} x | |
103 | (quote (1 2 3)) @result{} (1 2 3) | |
104 | (quote #(1 (2 3) 4)) @result{} #(1 (2 3) 4) | |
105 | @end example | |
106 | ||
107 | Note that an application must not attempt to modify literal lists or | |
108 | vectors obtained from a @code{quote} form, since they may be in | |
109 | read-only memory. | |
110 | ||
111 | @item (quasiquote @var{data}) | |
112 | @itemx `@var{data} | |
113 | @findex quasiquote | |
114 | @findex ` | |
115 | Backquote quasi-quotation is like @code{quote}, but selected | |
116 | sub-expressions are evaluated. This is a convenient way to construct | |
117 | a list or vector structure most of which is constant, but at certain | |
118 | points should have expressions substituted. | |
119 | ||
120 | The same effect can always be had with suitable @code{list}, | |
121 | @code{cons} or @code{vector} calls, but quasi-quoting is often easier. | |
122 | ||
123 | @table @nicode | |
124 | ||
125 | @item (unquote @var{expr}) | |
126 | @itemx ,@var{expr} | |
127 | @findex unquote | |
128 | @findex , | |
129 | Within the quasiquote @var{data}, @code{unquote} or @code{,} indicates | |
130 | an expression to be evaluated and inserted. The comma syntax @code{,} | |
131 | is simply a shorthand for an @code{unquote} form. For example, | |
132 | ||
133 | @example | |
134 | `(1 2 ,(* 9 9) 3 4) @result{} (1 2 81 3 4) | |
135 | `(1 (unquote (+ 1 1)) 3) @result{} (1 2 3) | |
136 | `#(1 ,(/ 12 2)) @result{} #(1 6) | |
137 | @end example | |
138 | ||
139 | @item (unquote-splicing @var{expr}) | |
140 | @itemx ,@@@var{expr} | |
141 | @findex unquote-splicing | |
142 | @findex ,@@ | |
143 | Within the quasiquote @var{data}, @code{unquote-splicing} or | |
144 | @code{,@@} indicates an expression to be evaluated and the elements of | |
145 | the returned list inserted. @var{expr} must evaluate to a list. The | |
146 | ``comma-at'' syntax @code{,@@} is simply a shorthand for an | |
147 | @code{unquote-splicing} form. | |
148 | ||
149 | @example | |
150 | (define x '(2 3)) | |
151 | `(1 ,@@x 4) @result{} (1 2 3 4) | |
152 | `(1 (unquote-splicing (map 1+ x))) @result{} (1 3 4) | |
153 | `#(9 ,@@x 9) @result{} #(9 2 3 9) | |
154 | @end example | |
155 | ||
156 | Notice @code{,@@} differs from plain @code{,} in the way one level of | |
157 | nesting is stripped. For @code{,@@} the elements of a returned list | |
158 | are inserted, whereas with @code{,} it would be the list itself | |
159 | inserted. | |
160 | @end table | |
161 | ||
162 | @c | |
163 | @c FIXME: What can we say about the mutability of a quasiquote | |
164 | @c result? R5RS doesn't seem to specify anything, though where it | |
165 | @c says backquote without commas is the same as plain quote then | |
166 | @c presumably the "fixed" portions of a quasiquote expression must be | |
167 | @c treated as immutable. | |
168 | @c | |
169 | ||
170 | @end table | |
171 | ||
172 | ||
173 | @node Comments | |
174 | @subsubsection Comments | |
175 | ||
176 | @c FIXME::martin: Review me! | |
177 | ||
178 | Comments in Scheme source files are written by starting them with a | |
179 | semicolon character (@code{;}). The comment then reaches up to the end | |
180 | of the line. Comments can begin at any column, and the may be inserted | |
181 | on the same line as Scheme code. | |
182 | ||
183 | @lisp | |
184 | ; Comment | |
185 | ;; Comment too | |
186 | (define x 1) ; Comment after expression | |
187 | (let ((y 1)) | |
188 | ;; Display something. | |
189 | (display y) | |
190 | ;;; Comment at left margin. | |
191 | (display (+ y 1))) | |
192 | @end lisp | |
193 | ||
194 | It is common to use a single semicolon for comments following | |
195 | expressions on a line, to use two semicolons for comments which are | |
196 | indented like code, and three semicolons for comments which start at | |
197 | column 0, even if they are inside an indented code block. This | |
198 | convention is used when indenting code in Emacs' Scheme mode. | |
199 | ||
200 | ||
201 | @node Block Comments | |
202 | @subsubsection Block Comments | |
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203 | @cindex multiline comments |
204 | @cindex block comments | |
205 | @cindex #! | |
206 | @cindex !# | |
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207 | |
208 | @c FIXME::martin: Review me! | |
209 | ||
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210 | In addition to the standard line comments defined by R5RS, Guile has |
211 | another comment type for multiline comments, called @dfn{block | |
212 | comments}. This type of comment begins with the character sequence | |
213 | @code{#!} and ends with the characters @code{!#}, which must appear on a | |
214 | line of their own. These comments are compatible with the block | |
215 | comments in the Scheme Shell @file{scsh} (@pxref{The Scheme shell | |
216 | (scsh)}). The characters @code{#!} were chosen because they are the | |
217 | magic characters used in shell scripts for indicating that the name of | |
218 | the program for executing the script follows on the same line. | |
219 | ||
220 | Thus a Guile script often starts like this. | |
221 | ||
222 | @lisp | |
223 | #! /usr/local/bin/guile -s | |
224 | !# | |
225 | @end lisp | |
226 | ||
227 | More details on Guile scripting can be found in the scripting section | |
228 | (@pxref{Guile Scripting}). | |
229 | ||
230 | ||
231 | @node Case Sensitivity | |
232 | @subsubsection Case Sensitivity | |
233 | ||
234 | @c FIXME::martin: Review me! | |
235 | ||
236 | Scheme as defined in R5RS is not case sensitive when reading symbols. | |
237 | Guile, on the contrary is case sensitive by default, so the identifiers | |
238 | ||
239 | @lisp | |
240 | guile-whuzzy | |
241 | Guile-Whuzzy | |
242 | @end lisp | |
243 | ||
244 | are the same in R5RS Scheme, but are different in Guile. | |
245 | ||
246 | It is possible to turn off case sensitivity in Guile by setting the | |
247 | reader option @code{case-insensitive}. More on reader options can be | |
248 | found at (@pxref{Reader options}). | |
249 | ||
250 | @lisp | |
251 | (read-enable 'case-insensitive) | |
252 | @end lisp | |
253 | ||
254 | Note that this is seldom a problem, because Scheme programmers tend not | |
255 | to use uppercase letters in their identifiers anyway. | |
256 | ||
257 | ||
258 | @node Keyword Syntax | |
259 | @subsubsection Keyword Syntax | |
260 | ||
261 | ||
262 | @node Reader Extensions | |
263 | @subsubsection Reader Extensions | |
264 | ||
265 | @deffn {Scheme Procedure} read-hash-extend chr proc | |
266 | @deffnx {C Function} scm_read_hash_extend (chr, proc) | |
267 | Install the procedure @var{proc} for reading expressions | |
268 | starting with the character sequence @code{#} and @var{chr}. | |
269 | @var{proc} will be called with two arguments: the character | |
270 | @var{chr} and the port to read further data from. The object | |
271 | returned will be the return value of @code{read}. | |
272 | @end deffn | |
273 | ||
274 | ||
275 | @node Scheme Read | |
276 | @subsection Reading Scheme Code | |
277 | ||
278 | @rnindex read | |
279 | @deffn {Scheme Procedure} read [port] | |
280 | @deffnx {C Function} scm_read (port) | |
281 | Read an s-expression from the input port @var{port}, or from | |
282 | the current input port if @var{port} is not specified. | |
283 | Any whitespace before the next token is discarded. | |
284 | @end deffn | |
285 | ||
286 | The behaviour of Guile's Scheme reader can be modified by manipulating | |
287 | its read options. For more information about options, @xref{User level | |
288 | options interfaces}. If you want to know which reader options are | |
289 | available, @xref{Reader options}. | |
290 | ||
291 | @c FIXME::martin: This is taken from libguile/options.c. Is there | |
292 | @c actually a difference between 'help and 'full? | |
293 | ||
294 | @deffn {Scheme Procedure} read-options [setting] | |
295 | Display the current settings of the read options. If @var{setting} is | |
296 | omitted, only a short form of the current read options is printed. | |
297 | Otherwise, @var{setting} should be one of the following symbols: | |
298 | @table @code | |
299 | @item help | |
300 | Display the complete option settings. | |
301 | @item full | |
302 | Like @code{help}, but also print programmer options. | |
303 | @end table | |
304 | @end deffn | |
305 | ||
306 | @deffn {Scheme Procedure} read-enable option-name | |
307 | @deffnx {Scheme Procedure} read-disable option-name | |
308 | @deffnx {Scheme Procedure} read-set! option-name value | |
309 | Modify the read options. @code{read-enable} should be used with boolean | |
310 | options and switches them on, @code{read-disable} switches them off. | |
311 | @code{read-set!} can be used to set an option to a specific value. | |
312 | @end deffn | |
313 | ||
314 | @deffn {Scheme Procedure} read-options-interface [setting] | |
315 | @deffnx {C Function} scm_read_options (setting) | |
316 | Option interface for the read options. Instead of using | |
317 | this procedure directly, use the procedures @code{read-enable}, | |
318 | @code{read-disable}, @code{read-set!} and @code{read-options}. | |
319 | @end deffn | |
320 | ||
321 | ||
322 | @node Fly Evaluation | |
323 | @subsection Procedures for On the Fly Evaluation | |
324 | ||
325 | @xref{Environments}. | |
326 | ||
327 | @rnindex eval | |
328 | @c ARGFIXME environment/environment specifier | |
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329 | @deffn {Scheme Procedure} eval exp module_or_state |
330 | @deffnx {C Function} scm_eval (exp, module_or_state) | |
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331 | Evaluate @var{exp}, a list representing a Scheme expression, |
332 | in the top-level environment specified by @var{module}. | |
333 | While @var{exp} is evaluated (using @code{primitive-eval}), | |
334 | @var{module} is made the current module. The current module | |
335 | is reset to its previous value when @var{eval} returns. | |
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336 | XXX - dynamic states. |
337 | Example: (eval '(+ 1 2) (interaction-environment)) | |
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338 | @end deffn |
339 | ||
340 | @rnindex interaction-environment | |
341 | @deffn {Scheme Procedure} interaction-environment | |
342 | @deffnx {C Function} scm_interaction_environment () | |
343 | Return a specifier for the environment that contains | |
344 | implementation--defined bindings, typically a superset of those | |
345 | listed in the report. The intent is that this procedure will | |
346 | return the environment in which the implementation would | |
347 | evaluate expressions dynamically typed by the user. | |
348 | @end deffn | |
349 | ||
350 | @deffn {Scheme Procedure} eval-string string [module] | |
351 | @deffnx {C Function} scm_eval_string (string) | |
352 | @deffnx {C Function} scm_eval_string_in_module (string, module) | |
353 | Evaluate @var{string} as the text representation of a Scheme form or | |
354 | forms, and return whatever value they produce. Evaluation takes place | |
355 | in the given module, or in the current module when no module is given. | |
356 | While the code is evaluated, the given module is made the current one. | |
357 | The current module is restored when this procedure returns. | |
358 | @end deffn | |
359 | ||
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360 | @deftypefn {C Function} SCM scm_c_eval_string (const char *string) |
361 | @code{scm_eval_string}, but taking a C string instead of an | |
362 | @code{SCM}. | |
363 | @end deftypefn | |
364 | ||
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365 | @deffn {Scheme Procedure} apply proc arg1 @dots{} argN arglst |
366 | @deffnx {C Function} scm_apply_0 (proc, arglst) | |
367 | @deffnx {C Function} scm_apply_1 (proc, arg1, arglst) | |
368 | @deffnx {C Function} scm_apply_2 (proc, arg1, arg2, arglst) | |
369 | @deffnx {C Function} scm_apply_3 (proc, arg1, arg2, arg3, arglst) | |
370 | @deffnx {C Function} scm_apply (proc, arg, rest) | |
371 | @rnindex apply | |
372 | Call @var{proc} with arguments @var{arg1} @dots{} @var{argN} plus the | |
373 | elements of the @var{arglst} list. | |
374 | ||
375 | @code{scm_apply} takes parameters corresponding to a Scheme level | |
376 | @code{(lambda (proc arg . rest) ...)}. So @var{arg} and all but the | |
377 | last element of the @var{rest} list make up | |
378 | @var{arg1}@dots{}@var{argN} and the last element of @var{rest} is the | |
379 | @var{arglst} list. Or if @var{rest} is the empty list @code{SCM_EOL} | |
380 | then there's no @var{arg1}@dots{}@var{argN} and @var{arg} is the | |
381 | @var{arglst}. | |
382 | ||
383 | @var{arglst} is not modified, but the @var{rest} list passed to | |
384 | @code{scm_apply} is modified. | |
385 | @end deffn | |
386 | ||
387 | @deffn {C Function} scm_call_0 (proc) | |
388 | @deffnx {C Function} scm_call_1 (proc, arg1) | |
389 | @deffnx {C Function} scm_call_2 (proc, arg1, arg2) | |
390 | @deffnx {C Function} scm_call_3 (proc, arg1, arg2, arg3) | |
8d596b11 | 391 | @deffnx {C Function} scm_call_4 (proc, arg1, arg2, arg3, arg4) |
07d83abe MV |
392 | Call @var{proc} with the given arguments. |
393 | @end deffn | |
394 | ||
395 | @deffn {Scheme Procedure} apply:nconc2last lst | |
396 | @deffnx {C Function} scm_nconc2last (lst) | |
397 | @var{lst} should be a list (@var{arg1} @dots{} @var{argN} | |
398 | @var{arglst}), with @var{arglst} being a list. This function returns | |
399 | a list comprising @var{arg1} to @var{argN} plus the elements of | |
400 | @var{arglst}. @var{lst} is modified to form the return. @var{arglst} | |
401 | is not modified, though the return does share structure with it. | |
402 | ||
403 | This operation collects up the arguments from a list which is | |
404 | @code{apply} style parameters. | |
405 | @end deffn | |
406 | ||
407 | @deffn {Scheme Procedure} primitive-eval exp | |
408 | @deffnx {C Function} scm_primitive_eval (exp) | |
409 | Evaluate @var{exp} in the top-level environment specified by | |
410 | the current module. | |
411 | @end deffn | |
412 | ||
413 | ||
414 | @node Loading | |
415 | @subsection Loading Scheme Code from File | |
416 | ||
417 | @rnindex load | |
ec3a8ace | 418 | @deffn {Scheme Procedure} load filename [reader] |
07d83abe | 419 | Load @var{filename} and evaluate its contents in the top-level |
ec3a8ace NJ |
420 | environment. The load paths are not searched. |
421 | ||
422 | @var{reader} if provided should be either @code{#f}, or a procedure with | |
423 | the signature @code{(lambda (port) @dots{})} which reads the next | |
424 | expression from @var{port}. If @var{reader} is @code{#f} or absent, | |
425 | Guile's built-in @code{read} procedure is used (@pxref{Scheme Read}). | |
426 | ||
427 | The @var{reader} argument takes effect by setting the value of the | |
428 | @code{current-reader} fluid (see below) before loading the file, and | |
429 | restoring its previous value when loading is complete. The Scheme code | |
430 | inside @var{filename} can itself change the current reader procedure on | |
431 | the fly by setting @code{current-reader} fluid. | |
432 | ||
433 | If the variable @code{%load-hook} is defined, it should be bound to a | |
434 | procedure that will be called before any code is loaded. See | |
435 | documentation for @code{%load-hook} later in this section. | |
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436 | @end deffn |
437 | ||
438 | @deffn {Scheme Procedure} load-from-path filename | |
439 | Similar to @code{load}, but searches for @var{filename} in the load | |
440 | paths. | |
441 | @end deffn | |
442 | ||
443 | @deffn {Scheme Procedure} primitive-load filename | |
444 | @deffnx {C Function} scm_primitive_load (filename) | |
445 | Load the file named @var{filename} and evaluate its contents in | |
446 | the top-level environment. The load paths are not searched; | |
447 | @var{filename} must either be a full pathname or be a pathname | |
448 | relative to the current directory. If the variable | |
449 | @code{%load-hook} is defined, it should be bound to a procedure | |
450 | that will be called before any code is loaded. See the | |
451 | documentation for @code{%load-hook} later in this section. | |
452 | @end deffn | |
453 | ||
40296bab KR |
454 | @deftypefn {C Function} SCM scm_c_primitive_load (const char *filename) |
455 | @code{scm_primitive_load}, but taking a C string instead of an | |
456 | @code{SCM}. | |
457 | @end deftypefn | |
458 | ||
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459 | @deffn {Scheme Procedure} primitive-load-path filename |
460 | @deffnx {C Function} scm_primitive_load_path (filename) | |
5c132e68 | 461 | Search @code{%load-path} for the file named @var{filename} and |
07d83abe MV |
462 | load it into the top-level environment. If @var{filename} is a |
463 | relative pathname and is not found in the list of search paths, | |
464 | an error is signalled. | |
465 | @end deffn | |
466 | ||
467 | @deffn {Scheme Procedure} %search-load-path filename | |
468 | @deffnx {C Function} scm_sys_search_load_path (filename) | |
5c132e68 | 469 | Search @code{%load-path} for the file named @var{filename}, |
07d83abe MV |
470 | which must be readable by the current user. If @var{filename} |
471 | is found in the list of paths to search or is an absolute | |
472 | pathname, return its full pathname. Otherwise, return | |
473 | @code{#f}. Filenames may have any of the optional extensions | |
474 | in the @code{%load-extensions} list; @code{%search-load-path} | |
475 | will try each extension automatically. | |
476 | @end deffn | |
477 | ||
ec3a8ace NJ |
478 | @defvar current-reader |
479 | @code{current-reader} holds the read procedure that is currently being | |
480 | used by the above loading procedures to read expressions (from the file | |
481 | that they are loading). @code{current-reader} is a fluid, so it has an | |
482 | independent value in each dynamic root and should be read and set using | |
483 | @code{fluid-ref} and @code{fluid-set!} (@pxref{Fluids and Dynamic | |
484 | States}). | |
485 | @end defvar | |
486 | ||
07d83abe | 487 | @defvar %load-hook |
42ad91f7 KR |
488 | A procedure to be called @code{(%load-hook @var{filename})} whenever a |
489 | file is loaded, or @code{#f} for no such call. @code{%load-hook} is | |
490 | used by all of the above loading functions (@code{load}, | |
491 | @code{load-path}, @code{primitive-load} and | |
492 | @code{primitive-load-path}). | |
493 | ||
494 | For example an application can set this to show what's loaded, | |
07d83abe MV |
495 | |
496 | @example | |
42ad91f7 KR |
497 | (set! %load-hook (lambda (filename) |
498 | (format #t "Loading ~a ...\n" filename))) | |
07d83abe | 499 | (load-from-path "foo.scm") |
42ad91f7 | 500 | @print{} Loading /usr/local/share/guile/site/foo.scm ... |
07d83abe | 501 | @end example |
07d83abe MV |
502 | @end defvar |
503 | ||
504 | @deffn {Scheme Procedure} current-load-port | |
505 | @deffnx {C Function} scm_current_load_port () | |
506 | Return the current-load-port. | |
507 | The load port is used internally by @code{primitive-load}. | |
508 | @end deffn | |
509 | ||
510 | @defvar %load-extensions | |
511 | A list of default file extensions for files containing Scheme code. | |
512 | @code{%search-load-path} tries each of these extensions when looking for | |
513 | a file to load. By default, @code{%load-extensions} is bound to the | |
514 | list @code{("" ".scm")}. | |
515 | @end defvar | |
516 | ||
517 | ||
518 | @node Delayed Evaluation | |
519 | @subsection Delayed Evaluation | |
520 | @cindex delayed evaluation | |
521 | @cindex promises | |
522 | ||
523 | Promises are a convenient way to defer a calculation until its result | |
524 | is actually needed, and to run such a calculation only once. | |
525 | ||
526 | @deffn syntax delay expr | |
527 | @rnindex delay | |
528 | Return a promise object which holds the given @var{expr} expression, | |
529 | ready to be evaluated by a later @code{force}. | |
530 | @end deffn | |
531 | ||
532 | @deffn {Scheme Procedure} promise? obj | |
533 | @deffnx {C Function} scm_promise_p (obj) | |
534 | Return true if @var{obj} is a promise. | |
535 | @end deffn | |
536 | ||
537 | @rnindex force | |
538 | @deffn {Scheme Procedure} force p | |
539 | @deffnx {C Function} scm_force (p) | |
540 | Return the value obtained from evaluating the @var{expr} in the given | |
541 | promise @var{p}. If @var{p} has previously been forced then its | |
542 | @var{expr} is not evaluated again, instead the value obtained at that | |
543 | time is simply returned. | |
544 | ||
545 | During a @code{force}, an @var{expr} can call @code{force} again on | |
546 | its own promise, resulting in a recursive evaluation of that | |
547 | @var{expr}. The first evaluation to return gives the value for the | |
548 | promise. Higher evaluations run to completion in the normal way, but | |
549 | their results are ignored, @code{force} always returns the first | |
550 | value. | |
551 | @end deffn | |
552 | ||
553 | ||
554 | @node Local Evaluation | |
555 | @subsection Local Evaluation | |
556 | ||
557 | [the-environment] | |
558 | ||
559 | @deffn {Scheme Procedure} local-eval exp [env] | |
560 | @deffnx {C Function} scm_local_eval (exp, env) | |
561 | Evaluate @var{exp} in its environment. If @var{env} is supplied, | |
562 | it is the environment in which to evaluate @var{exp}. Otherwise, | |
563 | @var{exp} must be a memoized code object (in which case, its environment | |
564 | is implicit). | |
565 | @end deffn | |
566 | ||
567 | ||
568 | @node Evaluator Behaviour | |
569 | @subsection Evaluator Behaviour | |
570 | ||
571 | @c FIXME::martin: Maybe this node name is bad, but the old name clashed with | |
572 | @c `Evaluator options' under `Options and Config'. | |
573 | ||
574 | The behaviour of Guile's evaluator can be modified by manipulating the | |
575 | evaluator options. For more information about options, @xref{User level | |
576 | options interfaces}. If you want to know which evaluator options are | |
577 | available, @xref{Evaluator options}. | |
578 | ||
579 | @c FIXME::martin: This is taken from libguile/options.c. Is there | |
580 | @c actually a difference between 'help and 'full? | |
581 | ||
582 | @deffn {Scheme Procedure} eval-options [setting] | |
583 | Display the current settings of the evaluator options. If @var{setting} | |
584 | is omitted, only a short form of the current evaluator options is | |
585 | printed. Otherwise, @var{setting} should be one of the following | |
586 | symbols: | |
587 | @table @code | |
588 | @item help | |
589 | Display the complete option settings. | |
590 | @item full | |
591 | Like @code{help}, but also print programmer options. | |
592 | @end table | |
593 | @end deffn | |
594 | ||
595 | @deffn {Scheme Procedure} eval-enable option-name | |
596 | @deffnx {Scheme Procedure} eval-disable option-name | |
597 | @deffnx {Scheme Procedure} eval-set! option-name value | |
598 | Modify the evaluator options. @code{eval-enable} should be used with boolean | |
599 | options and switches them on, @code{eval-disable} switches them off. | |
600 | @code{eval-set!} can be used to set an option to a specific value. | |
601 | @end deffn | |
602 | ||
603 | @deffn {Scheme Procedure} eval-options-interface [setting] | |
604 | @deffnx {C Function} scm_eval_options_interface (setting) | |
605 | Option interface for the evaluation options. Instead of using | |
606 | this procedure directly, use the procedures @code{eval-enable}, | |
607 | @code{eval-disable}, @code{eval-set!} and @code{eval-options}. | |
608 | @end deffn | |
609 | ||
610 | @c FIXME::martin: Why aren't these procedure named like the other options | |
611 | @c procedures? | |
612 | ||
613 | @deffn {Scheme Procedure} traps [setting] | |
614 | Display the current settings of the evaluator traps options. If | |
615 | @var{setting} is omitted, only a short form of the current evaluator | |
616 | traps options is printed. Otherwise, @var{setting} should be one of the | |
617 | following symbols: | |
618 | @table @code | |
619 | @item help | |
620 | Display the complete option settings. | |
621 | @item full | |
622 | Like @code{help}, but also print programmer options. | |
623 | @end table | |
624 | @end deffn | |
625 | ||
626 | @deffn {Scheme Procedure} trap-enable option-name | |
627 | @deffnx {Scheme Procedure} trap-disable option-name | |
628 | @deffnx {Scheme Procedure} trap-set! option-name value | |
629 | Modify the evaluator options. @code{trap-enable} should be used with boolean | |
630 | options and switches them on, @code{trap-disable} switches them off. | |
631 | @code{trap-set!} can be used to set an option to a specific value. | |
19ab431e HWN |
632 | |
633 | See @ref{Evaluator trap options} for more information on the available | |
634 | trap handlers. | |
07d83abe MV |
635 | @end deffn |
636 | ||
637 | @deffn {Scheme Procedure} evaluator-traps-interface [setting] | |
638 | @deffnx {C Function} scm_evaluator_traps (setting) | |
639 | Option interface for the evaluator trap options. | |
640 | @end deffn | |
641 | ||
642 | ||
643 | @c Local Variables: | |
644 | @c TeX-master: "guile.texi" | |
645 | @c End: |