[racket] procedure, contract, assoc, and REPL

Hi Robby -

What I am trying to do is to look up a key/value pair where the key happens
to be the procedure.  The struct idea will work only if I can serialize/hash
the procedure into a unique key, which means I will still have to rely on a
procedure matching itself (otherwise it's too much work for the client).

The scenario really boils down to - why does a contracted procedure not eq
itself?

The same script above shows that a contracted procedure returns #f with eq?,
but a plain procedure returns #t in REPL (both returns #t in module).

> (eq? test? test?) ;; test? is contracted.
#f
> (eq? test2? test2?) ;; test2? is not contracted
#t

Looking at the definition of
eq?<http://docs.racket-lang.org/reference/booleans.html#%28def._%28%28quote._~23~25kernel%29._eq~3f%29%29>shows
that eq? is for determining whether two objects are the same.  And
since both test? and test2? are exposed via a module, i.e. they are "fixed"
once exposed outside of module, whether contracted or not - I would have
expected eq? would return #t for both cases in REPL.

Am I misunderstand what eq? is all about?  Thanks.

yc

On Wed, Jul 21, 2010 at 5:40 PM, Robby Findler
<robby at eecs.northwestern.edu>wrote:

> You seem to be relying on identity of procedures (which is a bad
> idea). Contracts on functions are implemented with wrappers which
> breaks that by making fewer things equal but compiler optimizations
> can take things the other direction making more things equal than you
> might expect.
>
> Overall I think you should just avoid relying on the identity of
> procedures. One way to do that (without knowing more about your code)
> would be to use struct procedures and put an extra field on the struct
> that gives you the key for a comparison.
>
> Hth,
> Robby
>
> On Wednesday, July 21, 2010, YC <yinso.chen at gmail.com> wrote:
> > Hi all -
> >
> > I am running into an interesting issue where an alist of procedures in
> REPL, the ones without a contract works with assoc, but the ones with
> contract do not work with assoc.
> >
> > What I have is an alist where the keys are procedures.  Some of these
> procedures have contracts attached to them, and some do not.  Then I tried
> to use assoc to match the procedures.  If the code are run within the module
> itself - both types match, but within REPL only the ones without contract
> matches.
> >
> > Here's an simplified example, where foo.ss defines the procedures, and
> the test.ss uses foo.ss to construct the alist.
> >
> > ;; foo.ss
> > #lang scheme
> > (define (test? v) #t) ;; provided with contract
> >
> > (define (test2? v) #t) ;; provided without contract
> >
> > (provide/contract
> >  (test? (-> any/c boolean?))
> >  )
> >
> > (provide test2?)
> >
> > ;; test.ss
> > #lang scheme
> > (require "foo.ss")
> >
> > (define table `((,test2? . 1) (,test? . 2)))
> >
> > (assoc test? table) ;; works in module setting but not in REPL
> >
> > (assoc test2? table) ;; works in module setting AND in REPL
> >
> > Both the assoc test above returns the correct result.  But if I try to
> copy and run (assoc test? table) in REPL again, it will not match, whereas
> (assoc test2? table) continues to match.
> >
> > Is there any particular reason for the divergence of the behaviors?  I
> verified the behavior in both v4.2.5 and v5.0.  Thanks.
> >
> > yc
> >
> >
> >
> >
> >
>



-- 
Cheers,
yc

Taming the software dragon - http://dragonmaestro.com
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