In section 16.1.3 of the Racket Guide there is this example and explanation:
-----------------------------
(define-syntax clock
(syntax-id-rules (set!)
[(set! clock e) (put-clock! e)]
[(clock a ...) ((get-clock) a ...)]
[clock (get-clock)]))
(define-values (get-clock put-clock!)
(let ([private-clock 0])
(values (lambda () private-clock)
(lambda (v) (set! private-clock v)))))
The (clock a ...) pattern is needed because, when an identifier macro
is used after an open parenthesis, the macro transformer is given the
whole form, like with a non-identifier macro.
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If I run this in DrRacket and then type in the evaluation window:
> (clock)
It fails with "procedure application: expected procedure, given: 0 (no
arguments)" as it should.
If I now remove the line in the macro " [(clock a ...) ((get-clock) a ...)]"
then entering:either clock or (clock) at the evaluation prompt returns 0.
Why is it that (clock) doesn't still fail with "procedure
application: expected procedure, given: 0 (no arguments)" . I.e.
doesn't it expand to ((get-clock)).
Thanks,
Harry
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