1 ;;;; optargs.scm -- support for optional arguments
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44 ;;;; Contributed by Maciej Stachowiak <mstachow@alum.mit.edu>
50 ;;; {Optional Arguments}
52 ;;; The C interface for creating Guile procedures has a very handy
53 ;;; "optional argument" feature. This module attempts to provide
54 ;;; similar functionality for procedures defined in Scheme with
55 ;;; a convenient and attractive syntax.
57 ;;; exported macros are:
69 ;;; Summary of the lambda* extended parameter list syntax (brackets
70 ;;; are used to indicate grouping only):
72 ;;; ext-param-list ::= [identifier]* [#:optional [ext-var-decl]+]?
73 ;;; [#:key [ext-var-decl]+ [#:allow-other-keys]?]?
74 ;;; [[#:rest identifier]|[. identifier]]?
76 ;;; ext-var-decl ::= identifier | ( identifier expression )
78 ;;; The characters `*', `+' and `?' are not to be taken literally; they
79 ;;; mean respectively, zero or more occurences, one or more occurences,
80 ;;; and one or zero occurences.
85 (define-module (ice-9 optargs)
86 :export-syntax (let-optional
95 ;; let-optional rest-arg (binding ...) . body
96 ;; let-optional* rest-arg (binding ...) . body
97 ;; macros used to bind optional arguments
99 ;; These two macros give you an optional argument interface that is
100 ;; very "Schemey" and introduces no fancy syntax. They are compatible
101 ;; with the scsh macros of the same name, but are slightly
102 ;; extended. Each of binding may be of one of the forms <var> or
103 ;; (<var> <default-value>). rest-arg should be the rest-argument of
104 ;; the procedures these are used from. The items in rest-arg are
105 ;; sequentially bound to the variable namess are given. When rest-arg
106 ;; runs out, the remaining vars are bound either to the default values
107 ;; or to `#f' if no default value was specified. rest-arg remains
108 ;; bound to whatever may have been left of rest-arg.
111 (defmacro let-optional (REST-ARG BINDINGS . BODY)
112 (let-optional-template REST-ARG BINDINGS BODY 'let))
114 (defmacro let-optional* (REST-ARG BINDINGS . BODY)
115 (let-optional-template REST-ARG BINDINGS BODY 'let*))
119 ;; let-keywords rest-arg allow-other-keys? (binding ...) . body
120 ;; let-keywords* rest-arg allow-other-keys? (binding ...) . body
121 ;; macros used to bind keyword arguments
123 ;; These macros pick out keyword arguments from rest-arg, but do not
124 ;; modify it. This is consistent at least with Common Lisp, which
125 ;; duplicates keyword args in the rest arg. More explanation of what
126 ;; keyword arguments in a lambda list look like can be found below in
127 ;; the documentation for lambda*. Bindings can have the same form as
128 ;; for let-optional. If allow-other-keys? is false, an error will be
129 ;; thrown if anything that looks like a keyword argument but does not
130 ;; match a known keyword parameter will result in an error.
134 (defmacro let-keywords (REST-ARG ALLOW-OTHER-KEYS? BINDINGS . BODY)
135 (let-keywords-template REST-ARG ALLOW-OTHER-KEYS? BINDINGS BODY 'let))
137 (defmacro let-keywords* (REST-ARG ALLOW-OTHER-KEYS? BINDINGS . BODY)
138 (let-keywords-template REST-ARG ALLOW-OTHER-KEYS? BINDINGS BODY 'let*))
141 ;; some utility procedures for implementing the various let-forms.
143 (define (let-o-k-template REST-ARG BINDINGS BODY let-type proc)
144 (let ((bindings (map (lambda (x)
149 `(,let-type ,(map proc bindings) ,@BODY)))
151 (define (let-optional-template REST-ARG BINDINGS BODY let-type)
154 (let-o-k-template REST-ARG BINDINGS BODY let-type
158 ((not (null? ,REST-ARG))
159 (let ((result (car ,REST-ARG)))
160 ,(list 'set! REST-ARG
164 ,(cadr optional))))))))
166 (define (let-keywords-template REST-ARG ALLOW-OTHER-KEYS? BINDINGS BODY let-type)
169 (let* ((kb-list-gensym (gensym "kb:G"))
170 (bindfilter (lambda (key)
173 ((assq ',(car key) ,kb-list-gensym)
177 `(let* ((ra->kbl ,rest-arg->keyword-binding-list)
178 (,kb-list-gensym (ra->kbl ,REST-ARG ',(map
179 (lambda (x) (symbol->keyword (if (pair? x) (car x) x)))
181 ,ALLOW-OTHER-KEYS?)))
182 ,(let-o-k-template REST-ARG BINDINGS BODY let-type bindfilter)))))
185 (define (rest-arg->keyword-binding-list rest-arg keywords allow-other-keys?)
188 (let loop ((first (car rest-arg))
189 (rest (cdr rest-arg))
191 (let ((next (lambda (a)
192 (if (null? (cdr rest))
194 (loop (cadr rest) (cddr rest) a)))))
197 ((memq first keywords)
199 (error "Keyword argument has no value.")
200 (next (cons (cons (keyword->symbol first)
201 (car rest)) accum))))
202 ((not allow-other-keys?)
203 (error "Unknown keyword in arguments."))
204 (else (if (null? rest)
209 (loop (car rest) (cdr rest) accum)))))))
212 ;; lambda* args . body
213 ;; lambda extended for optional and keyword arguments
215 ;; lambda* creates a procedure that takes optional arguments. These
216 ;; are specified by putting them inside brackets at the end of the
217 ;; paramater list, but before any dotted rest argument. For example,
218 ;; (lambda* (a b #:optional c d . e) '())
219 ;; creates a procedure with fixed arguments a and b, optional arguments c
220 ;; and d, and rest argument e. If the optional arguments are omitted
221 ;; in a call, the variables for them are bound to `#f'.
223 ;; lambda* can also take keyword arguments. For example, a procedure
224 ;; defined like this:
225 ;; (lambda* (#:key xyzzy larch) '())
226 ;; can be called with any of the argument lists (#:xyzzy 11)
227 ;; (#:larch 13) (#:larch 42 #:xyzzy 19) (). Whichever arguments
228 ;; are given as keywords are bound to values.
230 ;; Optional and keyword arguments can also be given default values
231 ;; which they take on when they are not present in a call, by giving a
232 ;; two-item list in place of an optional argument, for example in:
233 ;; (lambda* (foo #:optional (bar 42) #:key (baz 73)) (list foo bar baz))
234 ;; foo is a fixed argument, bar is an optional argument with default
235 ;; value 42, and baz is a keyword argument with default value 73.
236 ;; Default value expressions are not evaluated unless they are needed
237 ;; and until the procedure is called.
239 ;; lambda* now supports two more special parameter list keywords.
241 ;; lambda*-defined procedures now throw an error by default if a
242 ;; keyword other than one of those specified is found in the actual
243 ;; passed arguments. However, specifying #:allow-other-keys
244 ;; immediately after the keyword argument declarations restores the
245 ;; previous behavior of ignoring unknown keywords. lambda* also now
246 ;; guarantees that if the same keyword is passed more than once, the
247 ;; last one passed is the one that takes effect. For example,
248 ;; ((lambda* (#:key (heads 0) (tails 0)) (display (list heads tails)))
249 ;; #:heads 37 #:tails 42 #:heads 99)
250 ;; would result in (99 47) being displayed.
252 ;; #:rest is also now provided as a synonym for the dotted syntax rest
253 ;; argument. The argument lists (a . b) and (a #:rest b) are equivalent in
254 ;; all respects to lambda*. This is provided for more similarity to DSSSL,
255 ;; MIT-Scheme and Kawa among others, as well as for refugees from other
259 (defmacro lambda* (ARGLIST . BODY)
262 (lambda (non-optional-args optionals keys aok? rest-arg)
263 ;; Check for syntax errors.
264 (if (not (every? symbol? non-optional-args))
265 (error "Syntax error in fixed argument declaration."))
266 (if (not (every? ext-decl? optionals))
267 (error "Syntax error in optional argument declaration."))
268 (if (not (every? ext-decl? keys))
269 (error "Syntax error in keyword argument declaration."))
270 (if (not (or (symbol? rest-arg) (eq? #f rest-arg)))
271 (error "Syntax error in rest argument declaration."))
272 ;; generate the code.
273 (let ((rest-gensym (or rest-arg (gensym "lambda*:G")))
274 (lambda-gensym (gensym "lambda*:L")))
275 (if (not (and (null? optionals) (null? keys)))
276 `(let ((,lambda-gensym
277 (lambda (,@non-optional-args . ,rest-gensym)
278 ;; Make sure that if the proc had a docstring, we put it
279 ;; here where it will be visible.
280 ,@(if (and (not (null? BODY))
281 (string? (car BODY)))
287 (let-keywords* ,rest-gensym
290 ,@(if (and (not rest-arg) (null? keys))
291 `((if (not (null? ,rest-gensym))
292 (error "Too many arguments.")))
296 (set-procedure-property! ,lambda-gensym 'arglist
303 `(lambda (,@non-optional-args . ,(if rest-arg rest-arg '()))
307 (define (every? pred lst)
309 (and (pred (car lst))
310 (every? pred (cdr lst)))))
312 (define (ext-decl? obj)
314 (and (list? obj) (= 2 (length obj)) (symbol? (car obj)))))
316 (define (parse-arglist arglist cont)
317 (define (split-list-at val lst cont)
321 (if (memq val (cdr pos))
322 (error (with-output-to-string
325 " specified more than once in argument list.")))))
326 (cont (reverse (cdr (memq val (reverse lst)))) (cdr pos) #t))))
327 (else (cont lst '() #f))))
328 (define (parse-opt-and-fixed arglist keys aok? rest cont)
331 (lambda (before after split?)
332 (if (and split? (null? after))
333 (error "#:optional specified but no optional arguments declared.")
334 (cont before after keys aok? rest)))))
335 (define (parse-keys arglist rest cont)
337 #:allow-other-keys arglist
338 (lambda (aok-before aok-after aok-split?)
339 (if (and aok-split? (not (null? aok-after)))
340 (error "#:allow-other-keys not at end of keyword argument declarations.")
343 (lambda (key-before key-after key-split?)
345 ((and aok-split? (not key-split?))
346 (error "#:allow-other-keys specified but no keyword arguments declared."))
349 ((null? key-after) (error "#:key specified but no keyword arguments declared."))
350 ((memq #:optional key-after) (error "#:optional arguments declared after #:key arguments."))
351 (else (parse-opt-and-fixed key-before key-after aok-split? rest cont))))
352 (else (parse-opt-and-fixed arglist '() #f rest cont)))))))))
353 (define (parse-rest arglist cont)
355 ((null? arglist) (cont '() '() '() #f #f))
356 ((not (pair? arglist)) (cont '() '() '() #f arglist))
357 ((not (list? arglist))
358 (let* ((copy (list-copy arglist))
359 (lp (last-pair copy))
362 (if (memq #:rest copy)
363 (error "Cannot specify both #:rest and dotted rest argument.")
364 (parse-keys copy ra cont))))
367 (lambda (before after split?)
370 ((0) (error "#:rest not followed by argument."))
371 ((1) (parse-keys before (car after) cont))
372 (else (error "#:rest argument must be declared last.")))
373 (parse-keys before #f cont)))))))
375 (parse-rest arglist cont))
379 ;; define* args . body
380 ;; define*-public args . body
381 ;; define and define-public extended for optional and keyword arguments
383 ;; define* and define*-public support optional arguments with
384 ;; a similar syntax to lambda*. They also support arbitrary-depth
385 ;; currying, just like Guile's define. Some examples:
386 ;; (define* (x y #:optional a (z 3) #:key w . u) (display (list y z u)))
387 ;; defines a procedure x with a fixed argument y, an optional agument
388 ;; a, another optional argument z with default value 3, a keyword argument w,
389 ;; and a rest argument u.
390 ;; (define-public* ((foo #:optional bar) #:optional baz) '())
391 ;; This illustrates currying. A procedure foo is defined, which,
392 ;; when called with an optional argument bar, returns a procedure that
393 ;; takes an optional argument baz.
395 ;; Of course, define*[-public] also supports #:rest and #:allow-other-keys
396 ;; in the same way as lambda*.
398 (defmacro define* (ARGLIST . BODY)
399 (define*-guts 'define ARGLIST BODY))
401 (defmacro define*-public (ARGLIST . BODY)
402 (define*-guts 'define-public ARGLIST BODY))
404 ;; The guts of define* and define*-public.
405 (define (define*-guts DT ARGLIST BODY)
406 (define (nest-lambda*s arglists)
409 `((lambda* ,(car arglists) ,@(nest-lambda*s (cdr arglists))))))
410 (define (define*-guts-helper ARGLIST arglists)
411 (let ((first (car ARGLIST))
412 (al (cons (cdr ARGLIST) arglists)))
414 `(,DT ,first ,@(nest-lambda*s al))
415 (define*-guts-helper first al))))
416 (if (symbol? ARGLIST)
417 `(,DT ,ARGLIST ,@BODY)
418 (define*-guts-helper ARGLIST '())))
422 ;; defmacro* name args . body
423 ;; defmacro*-public args . body
424 ;; defmacro and defmacro-public extended for optional and keyword arguments
426 ;; These are just like defmacro and defmacro-public except that they
427 ;; take lambda*-style extended paramter lists, where #:optional,
428 ;; #:key, #:allow-other-keys and #:rest are allowed with the usual
429 ;; semantics. Here is an example of a macro with an optional argument:
430 ;; (defmacro* transmorgify (a #:optional b)
432 (defmacro defmacro* (NAME ARGLIST . BODY)
433 (defmacro*-guts 'define NAME ARGLIST BODY))
435 (defmacro defmacro*-public (NAME ARGLIST . BODY)
436 (defmacro*-guts 'define-public NAME ARGLIST BODY))
438 ;; The guts of defmacro* and defmacro*-public
439 (define (defmacro*-guts DT NAME ARGLIST BODY)
441 (,(lambda (transformer) (defmacro:transformer transformer))
442 (lambda* ,ARGLIST ,@BODY))))
444 ;;; optargs.scm ends here