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. > >
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