[plt-scheme] Question on ill-defined contract

On Tue, 5 Jun 2007, Paulo J. Matos wrote:

> 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


I don't think this is broken; the above is a contract error!  We see the 
same issue with:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
(module foo mzscheme
   (require (lib "contract.ss"))
   (provide/contract [f (number? . -> . number?)])
   (define (f x)
     (* x x)))

(module bar mzscheme
   (require "foo.scm")
   (f 3 4))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


Here, we also see a similar error message:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
procedure f: expects 1 argument, given 2: 3 4
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


But this is a contract-level error: we can see this by wrapping a handler:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
(module bar mzscheme
   (require "foo.scm")

   (with-handlers ((exn:fail:contract?
                   (lambda (exn) (printf "contract error!~n"))))
     (f 3 4)))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

After which we see the "contact error!" message.  (Specifically, the 
exception raised is exn:fail:contract:arity).


Maybe you're asking for the default error display handler or the 
exception's exn-message to make it clear that a contract error? is going 
on?

Best of wishes!