[plt-scheme] mutual recursion in contracts

And there is one. flat-murec-contract.

Robby

At Fri, 7 May 2004 16:38:45 -0400, Matthias Felleisen wrote:
>   For list-related administrative tasks:
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> 
> That's not a question of lazy ornot, it's a question of providing a 
> form for mut rec types.
> 
> -- Matthias
> 
> On May 7, 2004, at 4:34 PM, David A. Herman wrote:
> 
> >  For list-related administrative tasks:
> >  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> >
> > I've noticed that contract expressions are evaluated immediately even 
> > though they aren't used until later. For example, the following code:
> >
> >     (define/contract x
> >       (begin
> >         (printf "evaluating contract!")
> >         integer?)
> >       'foo)
> >
> > outputs "evaluating contract!" and terminates successfully, but then 
> > any subsequent reference to x will fail with a contract error.
> >
> > This makes it hard to create syntactic forms that can produce 
> > definitions with mutually recursive contracts. Here's a simple 
> > example:
> >
> >     ;; an even is one of:
> >     ;;   - zero, i.e., (make-even #f)
> >     ;;   - successor(odd)
> >     ;; an odd is successor(even)
> >
> >     (define-type even ([pred (union odd? #f)]))
> >     (define-type odd ([pred even?]))
> >     =>
> >     (define-struct even (pred))
> >     (provide/contract (struct even ([pred odd?])))
> >     (define-struct odd (pred))
> >     (provide/contract (struct odd ([pred even?])))
> >
> > The above code causes an error because the provide/contract for even 
> > contains a forward reference to odd?. Of course, I can produce a 
> > letrec-like form so that I can reorder the expressions in the macro 
> > template, but in general I'd like to be able to have completely 
> > separate expressions with mutually recursive contracts.
> >
> > Clearly, in case contract expressions have side effects, you don't 
> > want to evaluate them multiple times. So it makes sense to evaluate 
> > them once. But would it make sense for contract expressions to be 
> > evaluated lazily, since they aren't needed at the time of the 
> > definition? I guess that's a little weird.
> >
> > Anyway, is there a good way for me to work around this? Am I stuck 
> > having to define a letrec-like form?
> >
> > Thanks,
> > Dave