[plt-scheme] contract question

(case (-> ..) (-> ..)) works perfect. Why did I miss that? It is in the docs, although at the very end of the chapter.

I did already go along the line of your example with ->*, but it is less restrictive than I want.

Thanks,
Jos
  ----- Original Message ----- 
  From: Dave Gurnell 
  To: plt-scheme at list.cs.brown.edu 
  Sent: Wednesday, February 06, 2008 2:04 PM
  Subject: Re: [plt-scheme] contract question


  I'm not sure why you can't use or/c (see below), but you should be able to use ->* or case->:


      (define (example a (b #f))
        (if b (+ a b) (void)))


  ->* specifies contracts for variable arity procedures:



      (provide/contract
       (example (->* (number?) (number?) (or/c number? void?))))


  case-> is intended for contracts on case-lambdas, but it should work okay here:


      (provide/contract
       (example (case-> (-> number? number? number?)
                        (-> number? void?))))


  I thought or/c could only be used on value contracts but the documentation suggests otherwise.


  Cheers,


  -- Dave


    Finally I decided to try contracts. I tried:

    #| require one or two numbers
       promise to return a number if two numbers are given
       promise void if one number is given |#

    #lang scheme

    (provide/contract
     (example
      (or/c
       (-> number? number? number?)
       (-> number? void?))))

    (define (example a (b #f))
     (if b (+ a b) (void)))

    This compiles, but it wont run:

    Welcome to DrScheme, version 3.99.0.10-svn4feb2008 [3m].
    Language: Module custom.
    or/c: two arguments, (-> number? void?) and (-> number? number? number?), might both match #<procedure:example>

    Why? What is wrong in my contract? May be I am wanting an impossible contract?
    or/c repeats exactly what I want, but does not accept it.
    I think I am missing something fundamental, but I can't find an answer in the docs.
    Thanks, Jos Koot
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