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))

result-expr

fail-expr)]

[(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

first-var

first-pattern

(minimatch1/fail rest-var rest-pattern result-expr fail-expr)

fail-expr))

fail-expr)]

[(minimatch1/fail v variable-id result-expr fail-expr)

(let ([variable-id v])

result-expr)]))