I'm a computer science student, programming language design
enthusiast, and occasional linguist/conlanger. mi'e Uroš Dimitrijević
Posts tagged: lambda
#lang racket
(define (>curry f . n-opt)
(let loop ((final-args '())
(n (if (empty? n-opt) 2 (car n-opt))))
(if (zero? n)
(apply f final-args)
(lambda nth-args
(loop (append final-args nth-args) (sub1 n))))))
(>curry list 0) ;=> '() ;; should this case really be…
((>curry list 1) 'a) ;=> '(a)
(((>curry list) 'a 'b 'c) 'd 'e 'f) ;=> '(a b c d e f)
((((>curry list 3) 'a 1) 'b 2) 'c 3) ;=> '(a 1 b 2 c 3)
(define (curry< f . n-opt)
(let loop ((final-args '())
(n (if (empty? n-opt) 2 (car n-opt))))
(if (zero? n)
(apply f final-args)
(lambda nth-args
(loop (append nth-args final-args) (sub1 n))))))
(curry< list 0) ;=> '() ;; …part of the interface, though?
((curry< list 1) 'a) ;=> '(a)
(((curry< list) 'a 'b 'c) 'd 'e 'f) ;=> '(d e f a b c)
((((curry< list 3) 'a 1) 'b 2) 'c 3) ;=> '(c 3 b 2 a 1)
#lang racket
;; This defines an n-argument version of the Y combinator. A use case is
;; implementing mutually recursive functions. The intended usage is:
;; ((y* (lambda (f1 f2 ... fn) (lambda <body of f1>))
;; (lambda (f1 f2 ... fn) (lambda <body of f2>))
;; ...
;; (lambda (f1 f2 ... fn) (lambda <body of fn>)))
;; (lambda (f1 f2 ... fn) <use of f1 to fn>))
;; Implementation:
(define u (lambda (f) (f f))) ;trusty old Mockingbird
(define y*
(lambda fs
(u (lambda (this)
(lambda (receiver)
(apply receiver
(map (lambda (f)
(lambda args
(apply ((u this) f) args)))
fs)))))))
;; An application: mutually recursive odd?/even? predicates
((y* (lambda (oddp evenp) (lambda (n) (if (zero? n) #f (evenp (sub1 n)))))
(lambda (oddp evenp) (lambda (n) (if (zero? n) #t (oddp (sub1 n))))))
(lambda (oddp evenp)
(list (oddp 5) (oddp 6) (evenp 7) (evenp 8))))
;; produces '(#t #f #f #t)
