Here's mine, probably not a very good way to implement this, but it
works and it uses the alt-exp syntax I've been playing with recently.
#lang alt-exp
#|
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)
TODO: add #' to alt-exp reader for (syntax ...)
|#
define-syntax: cas-cad-e-inner(stx)
syntax-case(stx [break]):
:[_ [prior ifs] e]
syntax:
ifs
:[_ [prior ifs] e [c s ...] ... [[cls] stm_0 stm ... break(v)]]
syntax:
let: :[action [lambda [] stm_0 stm ... v]]
cas-cad-e-inner:
[action if(equal?(e cls) action() ifs)]
e
[c s ...] ...
:[_ [prior ifs] e [c s ...] ... [[cls] stm_0 stm ...]]
syntax:
let: :[action [lambda [] stm_0 stm ... prior()]]
cas-cad-e-inner:
[action if(equal?(e cls) action() ifs)]
e
[c s ...] ...
define-syntax: cas-cad-e(stx)
syntax-case(stx [break]):
:[_ e]
syntax:
void()
:[_ e [c s ...] ... [[cls] stm_0 stm ... break(v)]]
syntax:
let: :[action [lambda [] stm_0 stm ... v]]
cas-cad-e-inner:
[action if(equal?(e cls) action() void())]
e
[c s ...] ...
:[_ e [c s ...] ... [[cls] stm_0 stm ...]]
syntax:
let: :[action [lambda [] stm_0 stm ...]]
cas-cad-e-inner:
[action if(equal?(e cls) action() void())]
e
[c s ...] ...
;;; TEST
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
_________________________________________________
For list-related administrative tasks:
http://lists.racket-lang.org/listinfo/users
