Re: [racket-users] Contract on a parameter’s value as a function precondition?

I think that Scott investigated adding support to chaperones that
would make something like this work.

Robby


On Tue, Nov 22, 2016 at 10:16 PM, Alexis King <[email protected]> wrote:
> I have a function that requires a parameter be set to a value satisfying
> a particular contract, but I don’t see any direct way to specify that
> constraint using the contract system. It seems possible to emulate using
> #:pre from ->* and ->i, but this has two problems: it doesn’t produce
> very good error messages, and it doesn’t let me provide an arbitrary
> contract.
>
> The latter issue seems a little trickier to implement, since I don’t
> think there’s any specific support in Racket’s existing interposition
> layer. Neither impersonate-procedure nor impersonate-procedure* provide
> any control over the dynamic extent of the call, so it isn’t possible to
> adjust the parameterization. It’s possible to create an extremely simple
> contract that creates an entirely new procedure instead of a chaperone
> or impersonator:
>
>   (define (parameterization-> param-name param val/c)
>     (make-contract
>      #:name `(parameterization-> ,param-name
>                                  ,(contract-name val/c))
>      #:projection
>      (λ (blame)
>        (let ([blame* (blame-add-context
>                       blame #:swap? #t
>                       (format "the value of (~a)" param-name))])
>          (λ (val)
>            (λ args
>              (parameterize ([param (((contract-projection val/c) blame*)
>                                     (param))])
>                (apply val args))))))))
>
> …but this seems like it probably has some drawbacks. Is there a better
> way to do this? And in any case, would this be something useful to add
> to ->* or ->i?
>
> Alexis
>
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