WHEN … EXCEPT

> (define-syntax when

(syntax-rules (except)

((_ condition form except c)

(if (and condition (not c))

(begin form)) )

((_ condition form ...)

(if condition (begin form ...)) )

) )

> (define n 18)

> (when (even? n)

(display 'not-prime) except (= n 2))

NOT-PRIME

> (set! n 2)

> (when (even? n)

(display 'not-prime) except (= n 2))

(nothing)

Note … must occur at the end of a list.

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