Hi Cistian,
Both of your examples are hygienic, in the sense that neither actually
do anything that violates macro hygiene.
When `a_definition` is defined in the module context (i.e., the
"global namespace"), it has the same scope as the `a_definition` used
inside `my-macro`. (There's more to it, but this is enough for now.)
When `a_definition` is defined in a local context (e.g., inside a
`(let () ...)` form) outside `my-macro`, it won't have the same scope
as the `a_definition` used inside `my-macro`.
There's an easy trick to get around this. As Simon suggested, make
`a_definition` an argument to `my-macro`:
(define-simple-macro (my-macro a_definition)
(display a_definition))
Now, each call to `my-macro` "borrows" `a_definition` from the macro
caller's scope. There are other ways to tackle the problem, but this
is the safest option.
Eric
On Mon, Oct 7, 2019 at 7:33 AM Cistian Alejandro Alvarado Vázquez
<alex.alvar...@resuelve.mx> wrote:
>
> Okay, that I am aware of, but why is it hygienic inside of let but *not* in
> the global namespace? Also, you should (from my limited understanding!) still
> be able to access bindings from inside of a macro, no? For example, I can
> call functions inside of a macro and those identifiers are available even
> though they weren't provided as arguments. So it appears that macros have
> access to module-level bindings but NOT more local bindings, and that is what
> I'm confused about.
>
> On Monday, 7 October 2019 08:04:56 UTC-5, Simon Schlee wrote:
>>
>> Your second macro does not work as you expect, because by default macros in
>> racket are hygienic, described roughly that means that a macro only
>> "manipulates" the syntax it is given as arguments, not with the syntax
>> around its invocation. As gfb has mentioned one way to do this is to use
>> parameters, this way you can achieve dynamic effects without breaking
>> hygiene.
>>
>> Why is hygiene good? This section in the guide explains:
>> https://docs.racket-lang.org/guide/pattern-macros.html?q=macro#%28part._.Lexical_.Scope%29
>>
>> You say you want to use some definitions in your macro, why not just pass
>> the relevant ones as parameters?
>> Without knowing more about what you are trying to do, it is difficult to
>> suggest a good direction/solution.
>>
>> Not part of the answer, but related and interesting:
>> (fifth RacketCon): Matthew Flatt — Bindings as Sets of Scopes
>> https://www.youtube.com/watch?v=ABWLveMNdzg
>
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