Re: define-module, #:export and export

[Maxime Devos]

On 04-01-2023 16:11, yarl baudig wrote:
> Hello guile.
>
> I don't know if that's a bug. Anyway, I am confused about this so I ask. I came across 
> this problem playing with guix source code. I will share different "tests" each 
> test is a directory with nothing but the files I share.
> each time the command to try the test (inside it's directory) is `guile 
> --no-auto-compile -L . main.scm`
> [...]
My (untested) hypothesis (*: things I'm unsure about)

If there is no #:export (...), then when
(define-operation (valid-path?)) is expanded, the identifier 
'valid-path?' is free (*).

Hence, the following ...

>         (syntax-rules (name ...)
>           ((_ name) id) ...)))))

refers to the free identifier 'valid-path?'.

Conversely, if there is #:export, then the (syntax-rules (name ...) ...) 
looks for the bound identifier 'valid-path?' (*).

Important: bound identifiers != free-identifier, even if they have the 
same symbol!  For (simple-format #t "~S\n" (operation-id valid-path?)) 
to work, the 'valid-path?' of that expression must be bound if the 
syntax-rules looks for a bound identifier, and free if it looks for a 
free identifier.

By adding a (export valid-path?) between the simple-format and the 
(define-operation ...), the 'valid-path?' in (simple-format ...) becomes 
a bound identifier (*).

Proposed solutions:

  (a) Move the (export ...) after the (simple-format ...) instead
      of in-between the define-operation and simple-format
  (b) Or put the uses of define-operation and define-enumeration-type
      in the same file.
  (c) Or use (eq? (syntax->datum ...) ...) instead of the
      'syntax-rules (...)', to ignore the lexical environment
      and hence the bound/free distinction.

      (This has consequences for users of 'main' that rename their
      import of 'valid-path?' -- by this change, operation-id will
      expect the unrenamed name, whereas in the original code (IIUC)
      it would expect the renamed name.)

Greetings,
Maxime.

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