Would syntax/parse have done better?
On Jan 3, 2012, at 2:15 PM, Eli Barzilay wrote: > Two hours ago, Marijn wrote: >> Hi, >> >> for/hash is giving me a syntax-error, but the error is in >> macro-expanded code which is not shown by default and which is also >> seemingly inaccessible via the macro-stepper. The for/hash >> expression works outside my macro, so I guess it's my fault, but it >> would help if the syntax-error could be a bit more informative. The >> code: [...] > > 1. The problem is not `for/hash' -- it's in your macro. You can see > that by replacing it, for example, with `let*' and the error > changes accordingly. > > 2. Minimizing the code in obvious ways got me down to this macro soup > which fails in the same way: > > (define-syntax dependent-boxes > (syntax-rules () > ((_ ((_variable_ . _rule_) ...)) > (let () > (define-syntax with-variables > (syntax-rules (_variable_ ...) > [(_ (a (... ...))) ((with-variables a) (... ...))] > [(_ _variable_) (list '_variable_)] > ... > [(_ non-variable) non-variable])) > (define-syntax rule-with-variables > (syntax-rules () > [(_ (rule)) (lambda () (with-variables rule))])) > (list (rule-with-variables _rule_) ...))))) > (dependent-boxes ((profit (let* ((y 1)) 2)))) > > Looks like the macro stepper does not step through local macros (at > least not in this case), but it did the first step of expanding the > `dependent-boxes' macro. > > 3. This leads to a smaller code that fails the same. > > (let () > (define-syntax with-variables > (syntax-rules (profit) > ((_ (a ...)) ((with-variables a) ...)) > ((_ profit) (list 'profit)) > ((_ non-variable) non-variable))) > (define-syntax rule-with-variables > (syntax-rules () ((_ (rule)) (lambda () (with-variables rule))))) > (list (rule-with-variables ((let* ((y 1)) 2))))) > > 4. It's now easy to continue reducing it to the core of the problem: > > (define-syntax with-variables > (syntax-rules () > ((_ (a ...)) ((with-variables a) ...)) > ((_ a) a))) > (with-variables (let* ((y 1)) 2)) > > Looks like you're doing a code walk (which is what makes me think > that this is too complex for whatever it is you're trying to do), > and treat some names in a special way. The problem is that > > ((with-variables a) ...) > > is creating a function application so the error is the same as just > entering `let*' to get its value -- and the only error that it can > throw at this point is a "bad syntax" with the whole expression, > which is just `let*'. > > 5. And BTW, the reason it doesn't work is that the first expansion > gets to > > ((with-variables let*) (with-variables ((y 1))) (with-variables 2)) > > and this form is not a macro -- so it's expanded as a function > call, > > (#%app ...same...) > > and the rest is obvious. > > -- > ((lambda (x) (x x)) (lambda (x) (x x))) Eli Barzilay: > http://barzilay.org/ Maze is Life! > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
