Thanks to this Eli, I now see what parameters are. By chance I also just came across your's, Ryan's and Matthew's article "Keeping It Clean with Syntax Parameters" http://www.schemeworkshop.org/2011/papers/Barzilay2011.pdf that makes crystal clear all the issues about the question I asked.
Cheers, Harry On Mon, May 21, 2012 at 10:01 AM, Eli Barzilay <e...@barzilay.org> wrote: > On Wednesday, Harry Spier wrote: >> [...] >> but I've been unable to create a macro to create this form: >> --------------------------------------- >> (with-sequence SOME-SEQUENCE >> . . . .. . (get-next) . >> . . . (get-next) . . . etc.) >> --------------------------------------- >> >> where I don't pass the names of the thunks to test for end of sequence >> or to get the next item in the sequence, but that they are called >> "more?" and "get-next" by convention. >> >> How would I do this. > > Since you're using the name as a function, you can do that easily with > a parameter: > > (define current-get-next (make-parameter (λ () (error "no next")))) > (define (get-next) ((current-get-next))) > (define-syntax-rule (with-sequence seq body ...) > (let-values ([(more? the-get-next) (sequence-generate seq)]) > (parameterize ([current-get-next the-get-next]) > body ...))) > (with-sequence '(1 2 3) > (list (get-next) (get-next) (get-next))) > > [You could also make it appear as a magical binding with an identifer > macro: > > (define-syntax next > (syntax-id-rules () > [(_ x ...) ((get-next) x ...)] > [_ (get-next)])) > ] > > This does have some performance penalty though, since each call to > `get-next' goes through the parameter. The way to avoid that is to > switch the parameter to a syntax parameter -- the idea is similar to a > parameter, except that it's done at the syntax level: > > (require racket/stxparam) > (define-syntax-parameter get-next > (λ (stx) (raise-syntax-error #f "not in `with-sequence'" stx))) > (define-syntax-rule (with-sequence seq body ...) > (let-values ([(more? the-get-next) (sequence-generate seq)]) > (syntax-parameterize ([get-next (make-rename-transformer > #'the-get-next)]) > body ...))) > (with-sequence '(1 2 3) > (list (get-next) (get-next) (get-next))) > > -- > ((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
