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February 15, 2008
Requiring up and down syntax levels
If you do any macro programming in PLT Scheme you are sure to run into the dreaded “no #%app syntax transformer is bound” error message at some point. Though puzzling, the fix is actually quite simple in almost all cases. Assuming you're using 3.99, you either need to:
(require (for-syntax scheme/base))(require (for-template scheme/base))
What the error means is that some syntax has expanded in a function application, but #%app, the PLT Scheme primitive that actually handles application, is not bound in the phase in which the syntax is being evaluated. Requiring for-syntax will bind #%app in the phase before the current evaluation phase, while requiring for-template will bind #%app in the phase after. In most cases you want for-syntax. However, if you are writing functions that return syntax that is then inserted into a program (such a function would be required for-syntax elsewhere) you must use the other form, to make sure the syntax has #%app available to it.
Posted by Noel at February 15, 2008 10:59 AM
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Comments
(define-syntax define
(let-syntax ((olddefine (syntax-rules () ((_ args ...) (define args ...)))))
((define args ...) (begin
(print 'hi)
(olddefine args ...)))))
Posted by: PJW at February 18, 2008 06:50 PM