Re: A lexical use-modules?

[Maxime Devos <maximedevos@telenet.be>]

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Stefan Israelsson Tampe schreef op zo 27-03-2022 om 16:37 [+0200]:
> I mean that modul-ref is a hash-table lookup, and that's more
> expensive as you say. I suggested that lexical variables should be
> let bound
> because you can deduce more things with them e.g. in
> 
> (let ((a (@ x y)))    
>     (when (vector? a) code ...))
> 
> Then a is known to be a vector and the check can be avoided inside
> code ...
> You suggest to use (@ x y) everywhere which is not terrible,
> especially with wingo's recent work. But still not sure that all
> optimisations work for referencing (@ x y) all the time. 

I could do this, but:

  * this won't work for macros, so I would have to make a special case
    for them

  * the imported variables might not yet have been initialised.  E.g.:

    (lambda ()
      (use-modules/lexical (foobar))
      a-foo ; initially #false
      (initialise-foobar!)
      a-foo) ; now 'footastic'

    The problem with your proposal is that modifications to global
    variables would not be detected --- the idea of 'use-modules' is to
    import the _variables_, not their current _values_.

    Additionally, doing the latter would be backwards-incompatible.

  * not all imported variables might be used, so your proposal might
    make things worse.  E.g.:

    (define (foo)
      (use-modules (ice-9 exceptions))
      (if (do-stuff)
          'ok
          (raise (make-some-exception with variables from (ice-9
exceptions)))))

   In the normal case, the variables would not need to be dereferenced.

  * I just want to make lexical use-modules work and I'm not currently
    interested in (possibly futily) trying to squeezing out some extra
    % or ‰ of performance.

> Would be nice though to be able to select the vars to be bound and
> improve on compilation speed if there is a lot of baggage (large
> module and only few var used)

When making a patch for merging 'use-module/lexical' into 'use-
modules', I intend to preserve support for #:select.

Greetings,
Maxime.

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