[racket] a little macro exercise
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