53 lines
1.3 KiB
Racket
53 lines
1.3 KiB
Racket
#lang s-exp "../cur.rkt"
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(require "sugar.rkt" "bool.rkt")
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;; TODO: override (all-defined-out) to enable exporting all these
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;; properly.
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(provide Nat z s add1 sub1 plus )
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(module+ test
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(require rackunit))
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(data Nat : Type
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(z : Nat)
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(s : (-> Nat Nat)))
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(define (add1 (n : Nat)) (s n))
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(module+ test
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(check-equal? (add1 (s z)) (s (s z))))
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(define (sub1 (n : Nat))
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(case n
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[z z]
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[(s (x : Nat)) IH: ((ih-n : Nat)) x]))
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(module+ test
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(check-equal? (sub1 (s z)) z))
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(define (plus (n1 : Nat) (n2 : Nat))
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(case n1
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[z n2]
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[(s (x : Nat))
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IH: ((ih-n1 : Nat))
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(s ih-n1)]))
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(module+ test
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(check-equal? (plus z z) z)
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(check-equal? (plus (s (s z)) (s (s z))) (s (s (s (s z))))))
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;; Credit to this function goes to Max
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(define (nat-equal? (n1 : Nat))
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(elim Nat n1 (lambda (x : Nat) (-> Nat Bool))
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(lambda (n2 : Nat)
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(elim Nat n2 (lambda (x : Nat) Bool)
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true
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(lambda* (x : Nat) (ih-n2 : Bool) false)))
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(lambda* (x : Nat) (ih : (-> Nat Bool))
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(lambda (n2 : Nat)
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(elim Nat n2 (lambda (x : Nat) Bool)
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false
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(lambda* (x : Nat) (ih-bla : Bool)
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(ih x)))))))
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(module+ test
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(check-equal? (nat-equal? z z) true)
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(check-equal? (nat-equal? z (s z)) false)
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(check-equal? (nat-equal? (s z) (s z)) true))
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