16.2.3 with-syntax and generate-temporaries

Since syntax-case lets us compute with arbitrary Scheme expression, we can more simply solve a problem that we had in writing define-for-cbr (see Extended Example: Call-by-Reference Functions), where we needed to generate a set of names based on a sequence id ...:

(define-syntax (define-for-cbr stx)

(syntax-case stx ()

[(_ do-f (id ...) body)

....

#'(define (do-f get ... put ...)

(define-get/put-id id get put) ...

body) ....]))

This example uses (define-syntax (id arg) body ...+), which is equivalent to (define-syntax id (lambda (arg) body ...+)).

In place of the ....s above, we need to bind get ... and put ... to lists of generated identifiers. We cannot use let to bind get and put, because we need bindings that count as pattern variables, instead of normal local variables. The with-syntax form lets us bind pattern variables:

(define-syntax (define-for-cbr stx)

(syntax-case stx ()

[(_ do-f (id ...) body)

(with-syntax ([(get ...) ....]

[(put ...) ....])

#'(define (do-f get ... put ...)

(define-get/put-id id get put) ...

body))]))

Now we need an expression in place of .... that generates as many identifiers as there are id matches in the original pattern. Since this is a common task, Scheme provides a helper function, generate-temporaries, that takes a sequece of identifiers and returns a sequence of generated identifiers:

(define-syntax (define-for-cbr stx)

(syntax-case stx ()

[(_ do-f (id ...) body)

(with-syntax ([(get ...) (generate-temporaries #'(id ...))]

[(put ...) (generate-temporaries #'(id ...))])

#'(define (do-f get ... put ...)

(define-get/put-id id get put) ...

body))]))

This way of generating identifiers is normally easier to think about than tricking the macro expander into generating names with purely pattern-based macros.

In general, the right-hand side of a with-handlers binding is a pattern, just like in syntax-case. In fact, a with-handlers form is just a syntax-case form turned partially inside-out.