Many thanks.
> On Jan 23, 2018, at 1:56 PM, Matthew Flatt <mfl...@cs.utah.edu> wrote: > > Sure, with whatever improvements you'd like to make. > > At 23 Jan 2018 16:53:15 -0500, "John Clements" wrote: >> Permission to paste this summary on Stack Exchange? >> >> >>> On Jan 23, 2018, at 1:20 PM, Matthew Flatt <mfl...@cs.utah.edu> wrote: >>> >>> At 23 Jan 2018 15:57:34 -0500, "'John Clements' via Racket Users" wrote: >>>> despite being inside of a binding of the name map >>> >>> That's the essence of the problem. Which things are in the scope of a >>> top-level definition? >>> >>> For example, is the reference to `f` in the scope of a binding of `f` >>> in >>> >>> (define (g x) (f x)) >>> (define (f x) x) >>> >>> ? >>> >>> How about in >>> >>> (begin >>> (define (g x) (f x)) >>> (define (f x) x)) >>> >>> ? >>> >>> Or in >>> >>> (expand '(define (f x) x)) >>> (define (g x) (f x)) >>> >>> or >>> >>> (begin >>> (expand '(define (f x) x)) >>> (define (g x) (f x))) >>> >>> ? >>> >>> The rule in Racket is that a top-level `define` doesn't change the >>> binding of an identifier until the `define` is evaluated. So, in >>> >>> (define (map x) ... map ...) >>> >>> the reference to `map` is expanded/compiled at a point where `map` >>> refers to a module import, not a variable named `map`. By the time the >>> definition is evaluated, it's too late to redirect the meaning of `map` >>> as a reference to an import. >>> >>> There are other choices, but I don't think there are going choices that >>> end up being significantly better or more consistent. The top level is >>> hopeless. >>> >>> Modules behave significantly better, in part because the scope of a >>> definition is clear: from the start of the module to the end. >>> >>>> When I try doing the same thing with a value defined in the empty program >>>> (with the name’f’), I see the error >>>> >>>> define-values: assignment disallowed; >>>> cannot re-define a constant >>>> constant: f >>>> >>>> Which leads me to the suspicion that maybe possibly this message is >> supposed >>>> to be shown for the re-definition of ‘map’, as well? >>> >>> The difference is that `f` starts out bound to the variable `f`, not to >>> an `f` import. For various reasons, we have decided that definitions >>> are allowed to shadow imports. >>> -- 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.
