Perfect. Thank you, Philip. The example definition is: (define-custom-hash-types string-hash #:key? string? string=? string-length)
'string-length' in the above is the "optional hash function". What does that mean? It's not demonstrated in the example code. On Tue, Oct 1, 2019 at 10:38 PM Philip McGrath <phi...@philipmcgrath.com> wrote: > > On Tue, Oct 1, 2019 at 10:21 PM David Storrs <david.sto...@gmail.com> wrote: >> >> For example, there's a hash, hasheqv, and hasheq. Is there a way to >> have a hash with a user-provided key-comparison function? > > > You probably want `define-custom-hash-types` and the related functions: > https://docs.racket-lang.org/reference/dicts.html#(form._((lib._racket%2Fdict..rkt)._define-custom-hash-types)) > More generally, you can of course use `gen:dict` to do exactly what you > want. There are corresponding options for sets, also. > >> >> Is it possible to put gen:equal+hash on something other than a struct? > > > In general, this would mean redefining how `equal?` works on built-in > datatypes, which would cause trouble for libraries that use those types. You > can choose what `equal?` means for datatypes you define, and you can choose > to use some other notion of equality on datatypes you're comparing, but you > can't mess with what `equal?` means when other people's code calls it on > other people's datatypes. > > One alternative that can sometimes make sense is creating a wrapper struct > that basically is just there to hang methods like `gen:equal+hash` on. > > -Philip > -- 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. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CAE8gKoevucB8Xem415T1T4sZwDqgQzwv-tuzPRO6DiFNhireXA%40mail.gmail.com.
