[plt-scheme] Question on ill-defined contract

Well, the message is instead "bar broke the contract on mzscheme's
application operator. It supplied a function that accepts three
arguments and only one argument to apply". Since the contract on
application isn't checked by the contract system, but a more primitive
layer, we get that error instead.

Robby

On 6/5/07, Paulo J. Matos <pocm at soton.ac.uk> wrote:
> On 6/5/07, Robby Findler <robby at cs.uchicago.edu> wrote:
> > It is a confusing (esp when you come to it from that angle!), yes, but
> > I think the behavior is correct. The contract only requires that the
> > function accept three arguments, which it does, so foo-fn does not
> > break its part of the contract. It does, however, require that callers
> > supply 3 arguments, which didn't happen.
> >
> > The confusing part is the error message -- the way the contract system
> > is implemented, the function that is actually provided by the foo
> > module only accepts 3 arguments.
> >
> > I'm not sure I can do better, tho. This is actually a really
> > interesting case. I'll have to think on it.
> >
>
> And is there anything in the contract implementation to make this
> happen only during a demonstration where everyone is trying to kill
> PLT Scheme?!? ;-)
>
> Anyway, thanks for the explanation. I thought that the foo-fn provided
> by the module would only accept 3 arguments due to contract
> specification. What I didn't really expect was the message to be so
> contract-less in terms of information, because if contract wraps
> foo-fn and exports it only to accept 3 arguments, the message expected
> would be that bar broke the contract of foo-fn when calling it with
> only 1 argument.
>
> > Robby
> >
> > On 6/5/07, Paulo J. Matos <pocm at soton.ac.uk> wrote:
> > > Hi all,
> > >
> > > Today while I was doing a demo on PLT Scheme I hit something I was a
> > > bit wordless (why does it happen every time were presenting
> > > something?).
> > > I defined a function (something along these lines):
> > > foo.scm
> > > (module foo mzscheme
> > >
> > >   (define (foo-fn a b c)
> > >       (+ a b c))
> > >
> > >   (provide/contract
> > >    [foo-fn (integer? integer? integer? . -> . integer?)]))
> > >
> > > Then since the only interesting parameter was a, b was by default 0
> > > and c was by default a + 5, I changed it to a opt-lambda:
> > >   (define foo-fn
> > >     (opt-lambda (a (b 0) (c (+ a 5)))
> > >       (+ a b c)))
> > >
> > > But I forgot to update the contract, when I went to show everyone from
> > > module bar, how opt-lambda would work nicely, I have:
> > > (module bar mzscheme
> > >   (require "foo.scm")
> > >   (foo-fn 5))
> > >
> > > and I get:
> > > procedure foo-fn: expects 3 arguments, given 1: 5
> > >
> > > The triggered question by the host of the demo was indeed intriguing.
> > > Shouldn't the contract be broken? Should I get some kind of error
> > > helping me to spot that the error was in the contract definition?
> > > When I click the bug I even get a decent error message. Is this a bug
> > > or a 'feature'?
> > >
> > > Cheers,
> > > --
> > > Paulo Jorge Matos - pocm at soton.ac.uk
> > > http://www.personal.soton.ac.uk/pocm
> > > PhD Student @ ECS
> > > University of Southampton, UK
> > > _________________________________________________
> > >   For list-related administrative tasks:
> > >   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> > >
> >
> >
> >
>
>
> --
> Paulo Jorge Matos - pocm at soton.ac.uk
> http://www.personal.soton.ac.uk/pocm
> PhD Student @ ECS
> University of Southampton, UK
>