What about reproviding foo and using contract-out to wrap foo in a new contract?
On Tuesday, September 4, 2018 at 9:00:06 PM UTC-4, David K. Storrs wrote: > > > > On Tue, Sep 4, 2018 at 8:55 PM, Matthew Butterick <m...@mbtype.com > <javascript:>> wrote: > >> >> On Sep 4, 2018, at 3:54 PM, David Storrs <david....@gmail.com >> <javascript:>> 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 racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
