I know about `curry` and `curryr`, of course. But is there a way to accomplish
arbitrary currying at run time (not just at the ends of the argument list)? (Or
is it called partial application?)
Let's suppose we have `f`:
(define f (λ (a b c d) (+ a b c d)))
(f 1 2 3 4) ; 10
And we want to reduce the arity of `f` by supplying 1 as the first arugment and
3 as the third. Maybe we write this like so:
(define g (reduce-arity f '(1 b 3 d)))
Where the '(1 b 3 d) notation matches the arity of `f`, and denotes "use 1 for
the first argument, 3 for the third argument, and then leave the other two
arguments alone." (This is not ideal notation because it precludes the use of
symbols as applied value, but let's leave that aside.)
So `g` would end up as a 2-arity function:
(define g (λ (b d) (f 1 b 3 d))
(g 2 4) ; 10
I came up with the idea below, but it seems brutal, even by my standards.
#lang racket
(require rackunit)
(define (reduce-arity proc pattern)
(unless (= (length pattern) (procedure-arity proc))
(raise-argument-error 'reduce-arity (format "list of length ~a, same as
procedure arity" (procedure-arity proc)) pattern))
(define reduced-arity-name (string->symbol (format "reduced-arity-~a"
(object-name proc))))
(define-values (id-names vals) (partition symbol? pattern))
(define new-arity (length id-names))
(procedure-rename
(λ xs
(unless (= (length xs) new-arity)
(apply raise-arity-error reduced-arity-name new-arity xs))
(apply proc (for/fold ([acc empty]
[xs xs]
[vals vals]
#:result (reverse acc))
([pat-item (in-list pattern)])
(if (symbol? pat-item)
(values (cons (car xs) acc) (cdr xs) vals)
(values (cons (car vals) acc) xs (cdr vals))))))
reduced-arity-name))
(define f (λ (a b c d) (+ a b c d)))
(define g (reduce-arity f '(1 b 3 d)))
(check-equal? (f 1 2 3 4) (g 2 4))
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