On Tue, Sep 4, 2018 at 8:55 PM, Matthew Butterick <[email protected]> wrote:
> > On Sep 4, 2018, at 3:54 PM, David Storrs <[email protected]> wrote: > > Say I have this (possibly from a third-party module): > > (define/contract (foo arg) (-> string? #t) #t) > > I want to ensure that the argument is always non-empty, so I tighten the > contract. I could do this: > > (set! foo (contract (-> non-empty-string? #t) foo 'foo 'neg)) > > Mutating the function is pretty ugly, but I'm not sure of a better way. > > > > Seems like this technique would break down with imported identifiers, for > instance: > > ;;;;;;;;;;;;;;;;;;;;; > > #lang racket > > (module foomod racket > (provide foo) > (define/contract (foo arg) (-> string? #t) #t)) > (require 'foomod) > > (set! foo (contract (-> non-empty-string? #t) foo 'foo 'neg)) > > (foo "") ; error: set!: cannot mutate module-required identifier in: foo > > > > Ah, good point. I'd realized that the tighter version would be visible only in the current module, but I hadn't noticed that you actually can't mutate imports. Thanks. > I am left with the following questions: > > 1) Is there a better way to do this? > > > Why not the old prefix-and-wrap? > > ;;;;;;;;;;;;;;;;;;;;; > > #lang racket > > (module foomod racket > (provide foo) > (define/contract (foo arg) (-> string? #t) #t)) > (require (prefix-in lax: 'foomod)) > > (define foo (contract (-> non-empty-string? #t) lax:foo 'foo 'neg)) > > (foo "") > > That works. > > 2) If I use this method in a module other than the one where foo was > defined, my expectation is that I would affect it only in the current > module but that other importers would not see the change. Is this right? > > > When you export this new `foo`, it carries the new contract, of course. > (The old one remains but because your wrapper contract is tighter, any > contract blame will happen in the new contract) > Great, thanks. This does exactly what I need. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
