109 lines
3.5 KiB
Scheme
109 lines
3.5 KiB
Scheme
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;;; These are some macros to support using regexp matching.
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;;; (let-match m mvars body ...)
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;;; Bind the vars in MVARS to the match & submatch strings of match data M,
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;;; and eval the body forms. #F is allowed in the MVARS list, as a don't-care
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;;; parameter.
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;;;
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;;; (if-match m mvars conseq alt)
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;;; The same as LET-MATCH -- eval the CONSEQ form in the scope of the
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;;; bound MVARS. However, if the match data M evaluates to false, instead
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;;; of blowing up, we execute the ALT form instead.
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(define-syntax let-match
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(lambda (exp r c)
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(if (< (length exp) 3)
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(error "No match-vars list in LET-MATCH" exp))
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(let ((m (cadr exp)) ; The match expression
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(mvars (caddr exp)) ; The match vars
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(body (cdddr exp)) ; The expression's body forms
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(%begin (r 'begin))
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(%match:substring (r 'match:substring))
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(%let* (r 'let*)))
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(cond ((null? mvars) `(,%begin ,@body))
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((pair? mvars)
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(let* ((msv (or (car mvars) (r 'match-val))) ; "match-struct var"
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(sm-bindings (let recur ((i 0) (vars (cdr mvars)))
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(if (pair? vars)
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(let ((var (car vars))
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(bindings (recur (+ i 1) (cdr vars))))
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(if var
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(cons `(,var (,%match:substring ,msv ,i))
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bindings)
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bindings))
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'()))))
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`(,%let* ((,msv ,m) ,@sm-bindings) ,@body)))
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(else (error "Illegal match-vars list in LET-MATCH" mvars exp))))))
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(define-syntax if-match
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(syntax-rules ()
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((if-match match-exp mvars on-match no-match)
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(cond (match-exp => (lambda (m) (let-match m mvars on-match)))
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(else no-match)))))
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;;; (MATCH-COND (<match-exp> <match-vars> <body> ...)
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;;; (TEST <exp> <body> ...)
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;;; (TEST <exp> => <proc>)
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;;; (ELSE <body> ...))
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;;;
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;;; The first clause is as-in IF-MATCH; the next three clauses are as-in COND.
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;;;
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;;; It would be slicker if we could *add* extra clauses to the syntax
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;;; of COND, but Scheme macros aren't extensible this way.
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;;; Two defs. The other expander produces prettier output -- one COND
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;;; rather than a mess of nested IF's.
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;(define-syntax match-cond
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; (syntax-rules (else test =>)
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; ((match-cond (else body ...) clause2 ...) (begin body ...))
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;
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; ((match-cond) (cond))
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;
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; ((match-cond (test exp => proc) clause2 ...)
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; (let ((v exp)) (if v (proc v) (match-cond clause2 ...))))
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;
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; ((match-cond (test exp body ...) clause2 ...)
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; (if exp (begin body ...) (match-cond clause2 ...)))
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;
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; ((match-cond (test exp) clause2 ...)
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; (or exp (match-cond clause2 ...)))
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;
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; ((match-cond (match-exp mvars body ...) clause2 ...)
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; (if-match match-exp mvars (begin body ...)
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; (match-cond clause2 ...)))))
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(define-syntax match-cond
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(syntax-rules ()
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((match-cond clause ...) (match-cond-aux () clause ...))))
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(define-syntax match-cond-aux
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(syntax-rules (test else)
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;; No more clauses.
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((match-cond-aux (cond-clause ...))
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(cond cond-clause ...))
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;; (TEST . <cond-clause>)
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((match-cond-aux (cond-clause ...)
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(test . another-cond-clause) clause2 ...)
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(match-cond-aux (cond-clause ... another-cond-clause)
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clause2 ...))
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;; (ELSE <body> ...)
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((match-cond-aux (cond-clause ...)
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(else body ...) clause2 ...)
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(match-cond-aux (cond-clause ... (else body ...))))
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;; (<match-exp> <mvars> <body> ...)
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((match-cond-aux (cond-clause ...)
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(match-exp mvars body ...) clause2 ...)
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(match-cond-aux (cond-clause ... (match-exp => (lambda (m)
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(let-match m mvars
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body ...))))
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clause2 ...))))
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