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Re: define-module, #:export and export

From: yarl baudig
Subject: Re: define-module, #:export and export
Date: Fri, 6 Jan 2023 14:31:43 +0100 (CET)

> That is also my understanding, confirmed by
> $ cat lib.scm
> (define-module (lib)
>    #:export (test))
> (define-syntax test
>    (lambda (sintax)
>      (syntax-case sintax ()
>        ((test id)
>         (datum->syntax sintax (free-identifier=? #'id #'thing))))))
> $ cat test.scm
> (define-module (main)
>    #:use-module (lib)
>    #:export (thing)
>    )
> (display (test thing))
> (newline)
> (define thing 5)
> $ guile3.0 -L . test.scm
> #f
> If you comment out #:export (thing), the result changes to #t.
> To put it perhaps more simply, the use of #:export causes Guile to 
> understand early that there will be a variable 'thing' in this module, 
> and makes the identifier 'thing' refer to this variable that is not yet 
> defined. However, hygiene implies that you want to be able to use 
> keywords as if they were not keywords if they are rebound, e.g. the 
> 'else' here doesn't cause the cond clause to be taken:
> (let ((else #f)) (cond (#f 'bla) (else 'foo) (#t 'bar)))
> $1 = bar
> The way this is done is by comparing with the original identifier given 
> to syntax-rules. Technically, they are compared with free-identifier=? . 
> This means that a use of the identifier matches the keyword iff both are 
> unbound, or both are bound to the same lexical binding. However, this 
> isn't the case here, as the keyword in the macro was unbound, but at the 
> point of use, it has been bound by #:export.
> Honestly, I think it is better to choose a different way of writing 
> these macros that avoids this confusing issue. Try defining the 
> operations at the same time as the enum so that the macro giving an enum 
> member refers to the bindings of the operators. If you give more context 
> on what you're trying to do, we could help more.
> Best,
> Jean

Thank you both. I am trying to process this. I (re-)read 6.8 Macros and read 
tspl4. I don't understand why you use "datum->syntax".
I heard your suggestion. I am only trying to grasp things for now. I realize I 
don't know enough (on guile and on guix) to go on a concrete goal yet.
I need to work more on this in order to understand but I wanted to thank you 
for answering that quickly!

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