How to map over values of a hash table (racket)

Question

I want to map a function over the values in a hash table, like so:

(hash-map add1 (hash "apple" 1 "pear" 2))
=> #hash(("apple" . 2) ("pear" . 3))

Is there a library function to do this? It'd be good to have one that worked on immutable hashetables too.

I looked on PlaneT, but didn't see anything there.

Now, if this really doesn't exist, I'll go ahead and write it. What would the etiquette for getting this into racket? I just fork it on github and add it to the standard library (and the docs!) and submit a pull request? Or should I make it a planeT first, and then ask for it to be moved in? I'd like to help, but I just don't know what's the 'proper' way to go about it.

Answer 1

There is a hash-map, but it returns a list [1]

You would have to write your own hash-map to do exactly what you want.

#lang racket

(define (new-hash-map f h)
  (make-immutable-hash (hash-map h (λ (k v) (cons k (f v))))))

(new-hash-map add1 (hash "apple" 1 "pear" 2))

; => '#hash(("pear" . 3) ("apple" . 2))

Another form you might be interested in is for/hash [2]:

#lang racket

(define (new-hash-map2 f h)
  (for/hash ([(k v) (in-hash h)]) (values k (f v))))

(new-hash-map2 add1 (hash "apple" 1 "pear" 2))

; => '#hash(("pear" . 3) ("apple" . 2))

If you feel that this functionality should be included with Racket, patches are most welcome! The best way to contribute is to fork on github and submit a pull request.