l...@gnu.org (Ludovic Courtès) writes:
> Hi Göran,
>
> Sorry for the delay.
>
> Göran Weinholt <go...@weinholt.se> skribis:
>
>> the case-lambda form is specified in r6rs-lib as accepting any number of
>> clauses, including zero. So this should not give an error:
>
> My interpretation of the ‘case-lambda’ implementation on p. 15 of
> r6rs-lib.pdf is that ‘case-lambda-help’ raises an assertion violation
> when ‘case-lambda’ is called with zero clauses.
The case-lambda-help macro is expanded from within
(lambda args
(let ((n (length args)))
(case-lambda-help args n
(fmls b1 b2 ...) ...)))
So, the full expansion is
(lambda args
(let ((n (length args)))
(assertion-violation #f "unexpected number of arguments")))
and thus a procedure that always returns an assertion violation.
>
> The text itself doesn’t explicitly mention that zero clauses are
> supported.
I would disagree with this. Even without looking at the implementation, you
see the specification of case-lambda as
(case-lambda <case-lambda clause> ...)
The traditional meaning of ..., as seen in syntax-rules, and elsewhere
in the r6rs, is 0 or more. Therefore a (case-lambda) form seems allowed
to me.
Oh, and an existence proof for good measure :)
scheme@(guile−user)> (import (rnrs))
scheme@(guile−user)> (define-syntax case-lambda
(syntax-rules ()
((_ (fmls b1 b2 ...))
(lambda fmls b1 b2 ...))
((_ (fmls b1 b2 ...) ...)
(lambda args
(let ((n (length args)))
(case-lambda-help args n
(fmls b1 b2 ...) ...))))))
scheme@(guile−user)> (define-syntax case-lambda-help
(syntax-rules ()
((_ args n)
(assertion-violation #f
"unexpected number of
arguments"))
((_ args n ((x ...) b1 b2 ...) more ...)
(if (= n (length ’(x ...)))
(apply (lambda (x ...) b1 b2 ...) args)
(case-lambda-help args n more ...)))
((_ args n ((x1 x2 ... . r) b1 b2 ...) more ...)
(if (>= n (length ’(x1 x2 ...)))
(apply (lambda (x1 x2 ... . r) b1 b2 ...)
args)
(case-lambda-help args n more ...)))
((_ args n (r b1 b2 ...) more ...)
(apply (lambda r b1 b2 ...) args))))
scheme@(guile−user)> (case-lambda)
$22 = #<procedure 905a980 at <current input>:734:0 args>
scheme@(guile−user)> ($22)
ERROR: ERROR: R6RS exception:
1. &assertion
2. &message: "unexpected number of arguments"
3. &irritants: ()
Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile−user) [1]> ,q
scheme@(guile−user)>
--
Ian Price
"Programming is like pinball. The reward for doing it well is
the opportunity to do it again" - from "The Wizardy Compiled"
