Thank you, thank you, thank you. I feel really stupid now :-) [[ I wrote this kind of macro a couple of weeks ago, and I continually ran into problems. Old man thinking gensym is generate temporary.]]
On Jul 6, 2012, at 2:59 PM, Matthew Flatt wrote: > I'll push improvements to the cross-reference and explanation in the docs. > > As I try to make an example illustrating problems, I see that Racket is > more resistant to problems created by `gensym' than I expected. In > principle, the following is a good example: > > a.rkt: > ------ > #lang racket > (define-syntax (define-and-provide stx) > (syntax-case stx () > [(_ id) > (with-syntax ([ga (gensym 'a)]) > #'(begin > (define ga (random)) > (define-syntax-rule (id) ga) > (provide id)))])) > (define-and-provide a) > > b.rkt: > ------ > #lang racket/base > (require "a.rkt") > (a) > > After `raco make b.rkt', the compiled form of "b.rkt" will refer to a > variable whose name is a gensym, and it will be a different gensym than > the one in "a.rkt" when the latter's code is loaded. The compiled code, > however, tracks both the name and relative (to other exports) position > for each variable reference; the linker is happy enough that the > printed forms of the names match up at the expected export position --- > which is a leftover from the days when `generate-temporaries' was > implemented with `gensym'. > > If you throw away the bytecode for "a.rkt" and try to load "b.rkt", > then you'll most likely get an error, and that error won't happen if > you use `generate-temporaries'. In other words, the real difference has > to do with determinism of expansion (as long as everything else in > expansion is deterministic). _________________________ Racket Developers list: http://lists.racket-lang.org/dev
