With a sufficiently clever compiler, that could be turned into (hash-ref hash key var) but I don't think that we have that clever a compiler. Without the ability to pass an explicit key like that at all, I think you'd be stuck, regardless.
Robby On Sun, Jul 18, 2010 at 3:55 PM, Jos Koot <jos.k...@telefonica.net> wrote: > Now consider: > > (let ((var ...)) > ... > (hash-ref hash key (lambda () var)) > > Do I understand well that this thunk is not cq cannot be compiled to a > constant? > When compiling the thunk, the location of var relative to the top of the > stack is known, is it not? > Thanks, > Jos > >> -----Original Message----- >> From: Matthew Flatt [mailto:mfl...@cs.utah.edu] >> Sent: 18 July 2010 22:43 >> To: Jos Koot >> Cc: 'Robby Findler'; 'plt-scheme' >> Subject: Re: [racket] hash-ref >> >> Actually, you're right. A `lambda' form like >> >> (lambda () #f) >> >> that doesn't close over anything will compile to a constant. >> >> There is some function-call overhead when crossing the >> boundary from the run-time system back to the JIT-code world, >> and that was partly the motivation for special-casing >> non-procedure arguments to `hash-ref'. >> The biggest reason, though, was that it's more convenient to pass `#f' >> than `(lambda () #f)', since a `#f' result is the most common case. >> >> At Sun, 18 Jul 2010 22:32:21 +0200, "Jos Koot" wrote: >> > Ha, that makes sense. >> > I had the (apparently wrong) idea that a thunk would be >> compiled to a >> > ready to use procedure and would be immediately available >> at run time. >> > Thanks. Jos >> > >> > > -----Original Message----- >> > > From: robby.find...@gmail.com >> > > [mailto:robby.find...@gmail.com] On Behalf Of Robby Findler >> > > Sent: 18 July 2010 15:39 >> > > To: Jos Koot >> > > Cc: plt-scheme >> > > Subject: Re: [racket] hash-ref >> > > >> > > It is for performance reasons. Specifically you can avoid >> creating >> > > the procedure over and over (if you're calling hash-ref over and >> > > over). >> > > >> > > Robby >> > > >> > > On Sunday, July 18, 2010, Jos Koot >> <jos.k...@telefonica.net> wrote: >> > > > >> > > > >> > > > >> > > > >> > > > >> > > > (hash-ref >> > > > a-hash a-key a-value) >> > > > In the past >> > > > a-value was required to be a procedure. >> > > > Now it can >> > > > be anything. >> > > > >> > > > Nice. >> > > > Two caveats >> > > > though. >> > > > 1. If the >> > > > value of a-value happens to be a procedure to be stored, it may >> > > > unintentionally be called. >> > > > 2. a-value >> > > > may be an expresssion that takes much computation. This >> > > computation is >> > > > repeated every time the hash-ref line is evaluated. >> > > > >> > > > Simply >> > > > helped of course: I make it a habbit always to write or to >> > > have macros >> > > > expand >> > > > to: >> > > > (hash-ref a-hash a-key (lambda () >> > > > a-value)) >> > > > >> > > > In short, I do not well understand the reason to relax the >> > > contract of >> > > > hash-ref and hash-ref!. >> > > > Jos >> > > > >> > > > >> > > > >> > >> > >> > _________________________________________________ >> > For list-related administrative tasks: >> > http://lists.racket-lang.org/listinfo/users > > > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
