Do checks like `hash?` in Racket support duck-typing?

Question

I'm a Python programmer new to Racket...

I have seen some code like:

(define table/c (and/c hash? (not/c immutable?)))

I understand this to be a use of Contracts, i.e. https://docs.racket-lang.org/reference/data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._and%2Fc%29%29

So it means that anywhere in this library which accepts a table/c arg, it should be an object which is a mutable hash-table.

My question is... does Racket have any support for duck-typing here (specifically to satisfy the contract of hash?)?

Say I wanted to replace the basic hash-table with something backed by a k/v store... Is there a 'hash-table' interface I could implement that would allow my store-backed custom object to pass a hash? contract check?

The docs for contracts and this for the hash? check didn't help me much.

Answer 1

I think you really want to use dictionaries for this, but if you're stuck with a library which wants hash? for contract checks then you can use impersonators.

Here's a function I wrote which tries to use a weak hash has a cache for some possibly-expensive computation represented by the (misnamed) filler function which will get called on a missing key.

I'm rather unsure about the semantics of this sort of thing (and I'm not an expert on any of this) but it works in practice.

(define (make-cache filler)
  (impersonate-hash
   (make-weak-hash '())
   (λ (ht key)
     ;; impersonate hash-ref by calling the filler
     (unless (hash-has-key? ht key)
       ;; I think this is safe as the key won't be dropped
       (hash-set! ht key (filler key)))
     (values key (λ (ht k v) v)))
   ;; Everything else is passed through
   (λ (ht k v)
     (values k v))
   (λ (ht k)
     k)
   (λ (ht k)
     k)))