I've been feeling pretty good about my macro-fu recently so I gave it a shot
(I havent looked at anyone's solutions yet). This seems to pass all the test
cases but I dont think it passes the "linear" requirement yet.
(define-syntax (cas-cad-e stx)
(syntax-case stx ()
[(_ e ((v) exp ...))
(with-syntax ([break (datum->syntax stx 'break)])
#'(call/cc
(λ (break)
(if (equal? e v)
(begin exp ...)
(void)))))]
[(_ e ((v1) exp1 ...) ((v2) exp2 ...) ...)
(with-syntax ([break (datum->syntax stx 'break)])
#'(call/cc
(λ (break)
(if (equal? e v1)
(begin exp1 ... exp2 ... ...)
(cas-cad-e e ((v2) exp2 ...) ...)))))]))
On Fri, Oct 8, 2010 at 9:04 PM, Shriram Krishnamurthi <s...@cs.brown.edu>
wrote:
> One of my students recently sent me this needless email message:
>
>> Well, how would you do switch fall-through in Scheme? Could you
>> write a version of the case statement that does that?
>
> Since the honor of Racket was at stake (yes, we can have all the same
> stupid features the scripting languages have!), I wrote down the code
> for this, and realized it would make a cute little macro exercise.
>
> Spec: define a case construct syntactically just like that of Racket.
> In terms of semantics:
>
> - each branch automatically falls through to the next,
>
> - the last one returns its answer since it has no next clause, and
>
> - any branch can contain (break <expr>), which evaluates <expr> and
> returns its value as that of the entire case.
>
> In honor of its behavor, we'll call this cas-cad-e. Thus,
>
> (define (cas1 v)
> (cas-cad-e v
> ((1) (display "1"))
> ((2) (display "2") (break 2)
> ((3) 3))))
>
> (cas1 1) ==> 2 (and prints "12")
> (cas1 2) ==> 2 (and prints "2")
> (cas1 3) ==> 3 (and prints nothing)
> (cas1 4) ==> <void> (and prints nothing)
>
> If anyone wants to look at my solution, here it is:
>
> http://github.com/shriram/cas-cad-e
>
> Of course, those who know how to do this don't need to peek, and those
> who don't shouldn't.
>
> Shriram
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