#|
ACL2's case-match, for destructuring s-expressions.
|#
(module case-match mzscheme
  (require (lib "plt-match.ss"))
  (require-for-template (lib "plt-match.ss"))
  
  (require-for-syntax (file "literal-identifier=.scm")
                      (file "syntax-checks.scm"))
  (provide case-match)
  
  ;; Produce #t iff identifier's name begins with #\!
  (define-for-syntax (starts-with-bang? id)
    (char=? #\! (string-ref (symbol->string (syntax-e id)) 0)))
  
  ;; identifier whose name starts with character #\! -> identifier
  (define-for-syntax (remove-bang !id)
    (let ([name (symbol->string (syntax-e !id))])
      (let ([new-name (substring name 1)])
        (datum->syntax-object !id (string->symbol new-name)))))
  
  (define-for-syntax (compile-pattern pat)
    (syntax-case* pat (quote & nil t) literal-identifier=?
      [& (syntax _)]
      [nil (syntax '())] ;; nil is implemented as Scheme null
      [t   (syntax 't)]
      [*const*
       (legal-constant-name? #'*const*)
       ;; NB:  Relies on (Scheme) equal? =/bool-rep (ACL2) equal
       (syntax (? (lambda (to-match) (equal? to-match *const*))))]
      [!name
       ;; NOTE:  The ACL2 pattern ! gets compiled to a match against whatever
       ;; is bound to the variable whose name is "".  This is consistent with ACL2.
       (and (identifier? #'!name)
            (starts-with-bang? #'!name))
       (with-syntax ([name (remove-bang #'!name)])
         (syntax (? (lambda (x) (equal? x name)))))]
      [var
       (identifier? #'var)
       (syntax var)]
      [(quote v) pat]
      [(p ...)
       (with-syntax ([(p* ...) (map compile-pattern 
                                    (syntax->list #'(p ...)))])
         (syntax (list p* ...)))]
      [(p q ... . r)
       (with-syntax ([(p* ...) (map compile-pattern
                                    (syntax->list #'(p q ...)))]
                     [r* (compile-pattern (syntax r))])
         (syntax (list-rest p* ... r*)))]
      [x #'x]
      ))
  (define-syntax (case-match stx)
    (syntax-case stx ()
      [(_ id)
       (unless (identifier? #'id)
         (raise-syntax-error #f "Expected an identifier" stx #'id))
       #'()]
      [(_ id [pat body] ...)
       (unless (identifier? #'id)
         (raise-syntax-error #f "Expected an identifier" stx #'id))
       (with-syntax ([(plt-pat ...) (map compile-pattern
                                         (syntax->list #'(pat ...)))])
         (if (memq '& (syntax-object->datum #'(pat ...)))
             #'(match id [plt-pat body] ...)
             #'(match id
                 [plt-pat body] ... 
                 [else '()])))]))
  )