31 lines
1.0 KiB
Racket
31 lines
1.0 KiB
Racket
#lang typed/racket/base
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(provide
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; call the updater on the dictionary key only if it has that key
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alist-maybe-update
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; update a value only if a condition succeeds on it
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u)
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(module+ test
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(require "typed-rackunit.rkt"))
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(: alist-maybe-update (∀ (A B) ((Listof (Pairof A B)) A (B -> B) -> (Listof (Pairof A B)))))
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(define (alist-maybe-update alist key updater)
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(map (λ ([p : (Pairof A B)])
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(if (eq? (car p) key)
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(cons (car p) (updater (cdr p)))
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p))
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alist))
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(module+ test
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(check-equal? (alist-maybe-update '((a . 5) (b . 6)) 'a (λ ([x : Number]) (+ x 10)))
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'((a . 15) (b . 6)))
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(check-equal? (alist-maybe-update '((b . 6)) 'a (λ ([x : Number]) (+ x 10)))
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'((b . 6))))
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(: u (∀ (A) ((A -> Any) (A -> A) A -> A)))
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(define (u condition updater value)
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(if (condition value) (updater value) value))
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(module+ test
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(check-equal? (u (λ ([x : Integer]) (< x 5)) (λ ([x : Integer]) (* x -1)) 4) -4)
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(check-equal? (u (λ ([x : Integer]) (< x 5)) (λ ([x : Integer]) (* x -1)) 8) 8))
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