3.5 Solution for Exercise 18

Like minimatch1, the minimatch macro needs a private variable for the value being matched. It needs a second private variable to represent the failure action. Consider the pattern (cons 'a 'b). The whole pattern can fail, because the value is not a pair; the first sub-pattern can fail; and the second sub-pattern can fail. There must be three occurrences of the same failure action—thus to avoid bad duplication, we must introduce a private variable.

The main macro, minimatch, just sets up the private variable for the value to be matched. The first helper macro, minimatch-clauses, recurs through the clauses and sets up private variables for failure actions. The second helper macro is similar to the helper macro for minimatch1, but it takes an extra argument, a failure expression, that it uses instead of directly calling error.

(define-syntax minimatch
  (syntax-rules ()
    [(minimatch val-expr clause ...)
     (let ([v val-expr])
       (minimatch-clauses v clause ...))]))
(define-syntax minimatch-clauses
  (syntax-rules ()
    [(minimatch-clauses v)
     (error 'minimatch "match failed")]
    [(minimatch-clauses v [pattern1 result-expr1] clause ...)
     (let ([fail (lambda () (minimatch* v clause ...))])
       (minimatch1/fail v pattern1 result-expr1 (fail)))]))
(define-syntax minimatch1/fail
  (syntax-rules (cons quote)
    [(minimatch1/fail v (quote datum) result-expr fail-expr)
     (if (equal? v (quote datum))
    [(minimatch1/fail v (cons first-pattern rest-pattern) result-expr fail-expr)
     (if (pair? v)
         (let ([first-var (car v)]
               [rest-var (cdr v)])
             (minimatch1/fail rest-var rest-pattern result-expr fail-expr)
    [(minimatch1/fail v variable-id result-expr fail-expr)
     (let ([variable-id v])