настало время факториалов.
--------------------------

мемоизация на ракете.
nothing special.

#lang racket
; replace define with a memoized version
(define-syntax define-memoized
(syntax-rules ()
[(_ (f args ...) bodies ...)
(define f
; store the cache as a hash of args => result
(let ([results (make-hash)])
; need to do this to capture both the names and the values
(lambda (args ...)
((lambda vals
; if we haven't calculated it before, do so now
(when (not (hash-has-key? results vals))
(hash-set! results vals (begin bodies ...)))
; return the cached result
(hash-ref results vals))
args ...))))]))

; example, fibonacci with memoization
(define-memoized (mfib n)
(cond
[(< n 1) 1]
[else (+ (mfib (- n 1)) (mfib (- n 2)))]))

#tsitfh

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