[racket] Why is x can be simultenaously bind and not bind ?

From: Jon Zeppieri (zeppieri at gmail.com)
Date: Sun Sep 15 19:21:18 EDT 2013

It may be helpful to look at the full expansion of your program. If
you open a fresh Dr. Racket buffer and enter the following program:

#lang racket/base

(let ([x 'uuu])
  (define x 'a)

Then, press the "Macro Stepper" button and, in the macro stepper
dialog, disable macro hiding (using the dropdown menu at the bottom),
and then press the "End" button, you'll see the following expansion:

(module anonymous-module racket/base
  (#%module-begin (#%app call-with-values (lambda () (let-values (((x)
'uuu)) (let-values (((x) 'a)) x))) print-values)))

The expansion makes explicit the nesting of lexical scopes.


On Sun, Sep 15, 2013 at 6:44 PM, Jon Zeppieri <zeppieri at gmail.com> wrote:
> On Sun, Sep 15, 2013 at 6:34 PM, Andre Mayers
> <Andre.Mayers at usherbrooke.ca> wrote:
>> Thank you for your answer. But it is still not clear.
>>> (let ([x 'uuu])
>>>   (define x 'a)
>>>   x)
>> This:
>> 1. creates a binding [x => 'uuu]
>> 2. creates a different binding [x => 'a] that shadows the existing binding
>> of x
>> 3. gives the value of the *visible* binding of x, namely 'a
>> The two binding [x => 'uuu] and [x =>'a] are exactly in the same lexical
>> space (the second binding is not in a lexical subspace) and that doesn't
>> make sense for me.
> No, they're not. As the reference says:
>      Definitions in an internal-definition context are equivalent
>      to local binding via letrec-syntaxes+values...[1]
>> As far as I know, to shadow a binding, you have to create
>> a lexical subspace. Am I wrong ?
> You're not wrong about that, but an internal definition expands into a
> letrec-syntaxes+values form, which does in fact create a new lexical
> scope.
> -Jon
> [1] http://docs.racket-lang.org/reference/syntax-model.html?q=internal#%28part._intdef-body%29

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