At Thu, 29 Jun 2017 09:32:24 -0700 (PDT), Sam Waxman wrote: > "The only way to check whether a form is an expression or definition is > to partially expand it and match on the form." > - This seems sort of rough to believe. If you shadow the syntax-rule for > define so that (define id binding) always just returns the number 1, then (+ > (define a 1) 1) will work perfectly, so somehow racket knows that the first > define is not an expression, whereas the second is. If you partially expand > it > and look at it's form, doesn't that just tell you that it expands into > something called define? Can't you not figure out if that's racket's define > or > a user-defined, expression define?
By "partial expansion", I meant expansion of the immediate form to a core form as in http://docs.racket-lang.org/reference/syntax-model.html?q=partial%20expansion#%28tech._partial._expansion%29 and as implemented by `local-expand` with a non-empty stop list. So, in your example, the expansion of `(define a 1)` won't stop there, since `define` doesn't have a core binding --- and if you make the example `define-values` instead of `define` as a macro that expands to `1`, that `define-values` still won't be bound to the core form. > "In this case, maybe you want to report the error from the > implementation `func` by using `syntax-local-context` to detect when > it's being used in an expression position." > > I just tried this and it worked pretty nicely! Thanks! Do you know if there's > any way to adapt this to let statements? I.e. > > (my-let () (func a(x) x)) > > Syntax-local-context just tells me that the func is in an internal-definition > position, but I'd like a way of saying "If the LAST body of the let is an > internal-definition, do something, and if not, do something else." No, there's not currently a way for a macro to know that expansion is in the last position with an internal-definition sequence. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
