[plt-scheme] Dynamic Contracts

See for a FAQ-organized introduction to contracts:

   -> http://www.ccs.neu.edu/home/matthias/Tmp/contracts.html

-- Matthias




On Jul 21, 2006, at 12:27 PM, Chris Warrington wrote:

> Does contract.ss support what I want to call "dynamic contracts?"
>
> Consider this function:
> ; make-list : natural-number X -> [X]
> ; Creates a list of the given length in which each element
> ; is the provided element.
> (define (make-list count val) ...)
>
> Using provide/contract (yes, I'm doing this in a module), I can  
> attach a simple
> contract to this:
> (provide/contract [make-list (natural-number/c any/c . -> . (listof  
> any/c))])
>
> However, this contract can be made stronger. For example, the  
> length of the list
> should be equal to count. However, I can't find a good way to do this.
> Thus far, the best I have is this:
>
> ; dc:make-list
> ; Dynamic contract for make-list
> (define (dc:make-list count val)
>   ((contract (natural-number/c any/c . -> . (and/c (listof any/c)
>                                                    (λ (lst) (=  
> (length lst)
> count))))
>              make-list
>              'callee
>              'caller)
>    count val))
>
> Is there an easier way to do these "dynamic contracts?" How would  
> one go about
> enforcing the type contraint in this case (i.e., the list should be  
> of the same
> type as val)?
>
> --
> Chris Warrington <chrisw at rice.edu>
> (away from his normal computer)
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