[racket] hash-ref

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:mflatt at 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.findler at gmail.com
> > > [mailto:robby.findler at 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.koot at 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
> > > >
> > > >
> > > >
> > 
> > 
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