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Re: Flush COMPILER-LET?
- To: Kent M Pitman <KMP@scrc-stony-brook.arpa>, sandra <@cs.utah.edu:sandra@defun>
- Subject: Re: Flush COMPILER-LET?
- From: Jeff Dalton <jeff%aiai.edinburgh.ac.uk@NSS.Cs.Ucl.AC.UK>
- Date: Tue, 27 Sep 88 15:12:02 BST
- Cc: email@example.com
- In-reply-to: Kent M Pitman's message of Mon, 26 Sep 88 16:48 EDT
> I think the intended use is slightly different from the example you gave.
> Here's what I think of as a prototypical example...
> (DEFVAR *LOCAL-TYPE-DECLARATIONS* '())
> (DEFMACRO LOCAL-TYPE-DECLARE (DECLARATIONS &BODY FORMS)
> `(COMPILER-LET ((*LOCAL-TYPE-DECLARATIONS*
> (APPEND ',DECLARATIONS *LOCAL-TYPE-DECLARATIONS*)))
> (DEFMACRO TYPED-VAR (VAR)
> (LET ((TYPE (ASSOC VAR *LOCAL-TYPE-DECLARATIONS*)))
> (IF TYPE `(THE ,(CADR TYPE) ,VAR) VAR)))
> (DEFUN F (X Y)
> (LOCAL-TYPE-DECLARE ((X FIXNUM) (Y FLOAT))
> (+ (TYPED-VAR X) (TYPED-VAR Y))))
> is equivalent to
> (DEFUN F (X Y) (+ (THE FIXNUM X) (THE FIXNUM Y)))
How about something like this:
(defun f (z)
(local-type-declare ((z fixnum))
#'(lambda () (typed-var z))))
If I then evaluate (F 10), I may get something like this:
#<%closure ((z 10)) ... more envs ... () (typed-var z)>
(TYPED-VAR Z) won't be expanded until I finally call this closure,
but by then the COMPILER-LET bindings have long since gone.
I think it was Cris Perdue who brought up this case in one of the
> You can do the same thing with MACROLET, but it feels quite awkward.
But it works.
> In my opinion, the only thing that makes COMPILER-LET feel bad is the
> stubborn refusal for interpreted-only implementations to do a semantic
> prepass. This is what forces you to do a special-bind at runtime. If
> COMPILER-LET did a special-bind only at conceptual compile time (i.e.,
> semantic pre-pass for interpreted code), it would not seem nearly so
> nasty because it wouldn't interfere unpredictably with runtime bindings.
What's required for the example above is for every interperter (not
just interpreter-only implementations) to do a semantic prepass.