Specializing functions accepting keywords

[Jens Axel Søgaard]

Consider the following fishy example wherein we try to
reuse a function with keyword arguments.


(define (fish #:name    [name   #f]
              #:color   [color  #f]
              #:studio  [studio #f]
              #:eyes    [eyes   2])
  (~a "The fish " name " is " color ", appears in a movie from " studio " and has " eyes " eyes."))

Let's check see an example:

> (fish #:name "Nemo"   #:color "orange" #:studio "Disney")
"The fish Nemo is orange, appears in a movie from Disney and has 2 eyes."

A few examples later one quickly realizes that Disney has
a lot of fish in their movies:


(fish #:name "Nemo"   #:color "orange" #:studio "Disney")
(fish #:name "Dory"   #:color "blue"   #:studio "Disney")
(fish #:name "Marlin" #:color "orange" #:studio "Disney")
(fish #:name "Wanda"  #:color "gold"   #:studio "MGM")
(fish #:name "Blinky" #:color "orange" #:studio "Fox" #:eyes 3)


No worries, we can quickly define a  disney-fish  that simply
calls  fish  using the #:studio keyword and passes other keywords
along. The new  disney-fish must accept the same keywords as fish
(except for the studio one), so  procedure-reduce-keyword-arity
is needed to restrict the accepted keywords.


(define disney-fish
  (let ()
    (define (make-disney-fish kws kw-args . rest)
      (keyword-apply/sort fish kws kw-args rest #:studio "Disney"))

    (define-values (fish-required fish-allowed) (procedure-keywords fish))
    (define fish-arity (procedure-arity fish)) ; number of by-position arguments

    (procedure-reduce-keyword-arity
       (make-keyword-procedure make-disney-fish)
       fish-arity
       (remove '#:studio fish-required)  
       (remove '#:studio fish-allowed))))

Now we can write:

    (disney-fish #:name "Nemo" #:color "orange")

The new  disney-fish  accepts only keywords accepted by fish,
so any error reporting works as expected.

But ... the above solution wasn't quick - it took more work than I initially expected.
Am I missing something that makes reusing functions accepting
keyword arguments easier?

/Jens Axel


-- full example --


#lang racket

(require kw-utils/keyword-apply-sort)

(define (fish #:name    [name   #f]
              #:color   [color  #f]
              #:studio  [studio #f]
              #:eyes    [eyes   2])
  (~a "The fish " name " is " color ", appears in a movie from " studio " and has " eyes " eyes."))

(fish #:name "Nemo"   #:color "orange" #:studio "Disney")
(fish #:name "Dory"   #:color "blue"   #:studio "Disney")
(fish #:name "Marlin" #:color "orange" #:studio "Disney")
(fish #:name "Blinky" #:color "orange" #:studio "Fox" #:eyes 3)

(define disney-fish
  (let ()
    (define (make-disney-fish kws kw-args . rest)
      (keyword-apply/sort fish kws kw-args rest #:studio "Disney"))
    (define-values (fish-required fish-allowed) (procedure-keywords fish))
    (define fish-arity (procedure-arity fish)) ; number of by-position arguments
    (procedure-reduce-keyword-arity
     (make-keyword-procedure make-disney-fish)
     fish-arity
     (remove '#:studio fish-required)
     (remove '#:studio fish-allowed))))
   

(disney-fish #:name "Nemo" #:color "orange"
             #:studio "foo")

; Notes:
; > (disney-fish #:name "Nemo" #:color "orange" #:studio "Disney")
; correctly shows error (#:studio not expected)

[Philip McGrath]

The easiest way I know is to use `curry`, since Alexis fixed its keyword argument support last year:

#lang racket

(define (fish #:name    [name   #f]
              #:color   [color  #f]
              #:studio  [studio #f]
              #:eyes    [eyes   2])
  (~a "The fish " name " is " color ", appears in a movie from " studio " and has " eyes " eyes."))

(define disney-fish
  (curry fish #:studio "Disney"))

(disney-fish #:name "Nemo" #:color "orange")

… but it might be useful to have some other library functions for this sort of thing: maybe a "curry once" or something like the `adjust-keyword-default` used in the `#lang slideshow` implementation.

-Philip

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