35 lines
1.0 KiB
Racket
35 lines
1.0 KiB
Racket
#lang typed/racket/base
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(provide list-union list-difference unique)
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(: list-union ((Listof Symbol) (Listof Symbol) -> (Listof Symbol)))
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(define (list-union s1 s2)
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(cond [(null? s1) s2]
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[(memq (car s1) s2)
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(list-union (cdr s1) s2)]
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[else (cons (car s1) (list-union (cdr s1) s2))]))
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(: list-difference ((Listof Symbol) (Listof Symbol) -> (Listof Symbol)))
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(define (list-difference s1 s2)
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(cond [(null? s1) '()]
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[(memq (car s1) s2)
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(list-difference (cdr s1) s2)]
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[else
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(cons (car s1) (list-difference (cdr s1) s2))]))
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;; Trying to work around what looks like a bug in typed racket:
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(define string-sort (inst sort String String))
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(: unique ((Listof Symbol) -> (Listof Symbol)))
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(define (unique los)
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(let: ([ht : (HashTable Symbol Boolean) (make-hasheq)])
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(for ([l los])
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(hash-set! ht l #t))
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(map string->symbol
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(string-sort
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(hash-map ht (lambda: ([k : Symbol] [v : Boolean])
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(symbol->string k)))
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string<?)))) |