I think its because without the (clock a ...) pattern the input (clock) is matched to the pattern 'clock' which matches by default. The pattern for just a lone identifier should be (in terms of syntax/parse) 'clock:identifier'. The docs for make-set!-transformer suggest to use a fender expression to check that the 'clock' pattern really matched an identifier, but since you don't have fenders in `syntax-id-rules' you either just assume the single identifier clause is the only possible choice remaining, or you can check it in the template and throw a syntax-error if its not an identifier.
Also that example is somewhat bizarre.. I think the (clock a ...) template should be (get-clock) instead of ((get-clock) a ...), right? On 06/04/2012 09:28 PM, Harry Spier wrote: > 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. > ------------------------------------------------------- > 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 > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
