[PRL] compiling `return'

[John Clements]

On Jun 22, 2005, at 9:33 PM, Dave Herman wrote:

> If you're compiling an imperative language with `return' into Scheme, 
> is the easiest implementation of tail call elimination to capture an 
> escape continuation (or raise an exception) that expects a thunk and 
> then apply the thunk in tail position? E.g.:
>
>     [[ proc(x1 .. xn) { ... return e; ... } ]]
>   = (lambda (x1 .. xn)
>       (let ([thunk (let/ec return
>                      ...
>                      (return (lambda () [[ e ]]))
>                      ...)])
>         (thunk)))

I remember talking with Joe Marshall about this at one point.  It seems 
to me that the "nice" thing to do would be to compile the code in such 
a way that no escape continuation was necessary.  That is, the return 
should be the end of the control flow for the procedure.  In the 
simplest case

(begin M0 M1 (if (= x 0) (return e)) M2 ...)

becomes

(begin M0 M1 (if (= x ) e (begin M2 ...)))

I claim that you can always perform such a translation; it gets most 
painful in the presence of macros that expand into lambda's, where the 
(return) should end not just one procedure but many of them, in which 
case you wind up doing a partial CPS.

On the other hand, perhaps escape continuations are just as fast.  I 
think Joe would know more about this.

John