24 lines
798 B
Scheme
24 lines
798 B
Scheme
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; pattern-lambda syntax for jscheme
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; pattern-lambda abstraction
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; this is a generalization of lambda:
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;
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; ((pattern-lambda p body) expr)
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; Matches expr against p. If no match, return #null. If match succeeds, evaluate body
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; with variables in p bound to whatever they matched in expr.
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;
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; EXAMPLE: Recognize adding any expression x to itself, replace with 2*x.
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; (define selfadd (pattern-lambda (+ x x) `(* 2 ,x)))
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; Then (selfadd '(+ (foo bar) (foo bar))) returns (* 2 (foo bar))
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;
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(define-macro (pattern-lambda pat body)
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(let* ((args (patargs pat))
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(expander `(lambda ,args ,body)))
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`(lambda (expr)
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(let ((m (match ',pat expr)))
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(if m
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; matches; perform expansion
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(apply ,expander (map (lambda (var) (cdr (or (assq var m) '(0 . #f))))
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',args))
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#f)))))
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