On Thu, Dec 1, 2016 at 7:29 PM, David Storrs <[email protected]> wrote:
> I'm having trouble understanding the docs on hash contracts ( > https://docs.racket-lang.org/reference/data-structure- > contracts.html#%28def._%28%28lib._racket%2Fcontract% > 2Fprivate%2Fhash..rkt%29._hash%2Fc%29%29) > > What I'm trying to express is: > > - This function returns #t because it is a simple test function intended > to get the hang of hash contracts... > - This function takes one argument... > - Which is a hash... > - Which has keys 'success, 'file-id, 'scratchdir-path, and 'chunk-hash... > - All of which are symbols... > - The values will be, respectively: > success boolean? > 1. Like Alexis says, `hash/dc` lets you specify a relationship between keys and values **but** the same relationship needs to hold for every key/value pair. Here's an example: #lang racket (require rackunit) (define my-hash/c (hash/dc [k char?] [v (k) (=/c (char->integer k))])) (contract my-hash/c (make-hash '((#\A . 65) (#\B . 66))) '+ '-) ;; good hash (contract my-hash/c (make-hash '((#\A . 66))) 'pos 'neg) ;; bad hash 2. And here's a funny way to check the property you care about (define special-list/c (list/c (cons/c 'chunk-hash string?) (cons/c 'file-id (or/c #f exact-positive-integer?)) (cons/c 'scratchdir-path path-string?) (cons/c 'success boolean?))) (define (special-hash/c h) (special-list/c (sort (hash->list h) symbol<? #:key car))) ;; example (contract special-hash/c (make-hash (list (cons 'file-id #f) (cons 'success #t) (cons 'chunk-hash "") (cons 'scratchdir-path (find-system-path 'temp-dir)))) '+ '-) -- 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.
