ref: 9acdf313b977ce11cf05254f9f05f6f8725ffbaa
dir: /femtolisp/ast/plambda-js.scm/
; pattern-lambda syntax for jscheme
; pattern-lambda abstraction
; this is a generalization of lambda:
;
; ((pattern-lambda p body) expr)
; Matches expr against p. If no match, return #null. If match succeeds, evaluate body
; with variables in p bound to whatever they matched in expr.
;
; EXAMPLE: Recognize adding any expression x to itself, replace with 2*x.
; (define selfadd (pattern-lambda (+ x x) `(* 2 ,x)))
; Then (selfadd '(+ (foo bar) (foo bar))) returns (* 2 (foo bar))
;
(define-macro (pattern-lambda pat body)
(let* ((args (patargs pat))
(expander `(lambda ,args ,body)))
`(lambda (expr)
(let ((m (match ',pat expr)))
(if m
; matches; perform expansion
(apply ,expander (map (lambda (var) (cdr (or (assq var m) '(0 . #f))))
',args))
#f)))))