On Thu 16 Feb 2012 14:18, Noah Lavine <noah.b.lav...@gmail.com> writes:
>> (let ((x (random)))
>> (eq? x x))
>
> (let* ((x (random))
> (y (list x))
> (z (car y))
> (eq? x z))
This one should reduce to the same thing, but currently doesn't because
(list x) is not considered a "constant expression" because `let' is a
"constructor" (quotes to indicate the language of peval). If we can
propagate it (probably true in this case, given that it has only one
use), then the expression folds as before:
(let* ((x (random))
(z (car (list x))))
(eq? x z))
=>
(let ((x-129 (random)))
(eq? x-129 x-129)))
So I don't think that for this purpose (identity) we need any more
mechanism
> In order to type-check, you need to be able to associate extra
> information with a value (its type) even in cases where you don't know
> the value.
For this purpose, have you considered creating a functional database of
facts? Then you can add facts when you see conditionals, for example,
and you get different facts in the different branches.
(if test consequent alternate)
;; assuming the test doesn't fold
=> (make-if (for-test test current-facts)
(for-tail consequent (add-fact test #t current-facts))
(for-tail consequent (add-fact test #f current-facts)))
You can also join the facts accumulated by a conditional branch with the
current facts if you detect that one of the branches leads to a nonlocal
exit, as in
(define (foo x)
(if (not (and (struct? x) (eq? (struct-vtable x) <foo>)))
(error ...))
;; here at this point, we have the fact: x is a struct, and its
;; vtable is <foo>
...)
This is related to range analysis. But, it's also something that's
easier to do with an explicit notion of control flow (i.e. a CPS IR).
Regards,
Andy
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