Re: [d.love@dl.ac.uk: dynamic loading of native code modules]

[Marius Vollmer <mvo@zagadka.ping.de>]

Neil Jerram <neil@ossau.uklinux.net> writes:

> If we can agree this, it would be good to do it in 1.6,

No. :-)

>     Thien-Thi> alternatively, we need to document *why* 1.6 chooses
>     Thien-Thi> to rob the users so, at least to ourselves.  "This
>     Thien-Thi> has been found to be too tricky, and is no longer
>     Thien-Thi> supported" is, although not dis-honest, still pretty
>     Thien-Thi> lame.
> 
> Upon reflection, I agree.

Yeah, ok.  Let me try to explain.  (This is the new
workbook/modules/modules-and-shared-libs.text.)

Guile used to be able to automatically find and link a shared
library to satisfy requests for a module.  For example, the module
`(foo bar)' could be implemented by placing a shared library named
"foo/libbar.so" (or with a different extension) in a directory on the
load path of Guile.

This has been found to be too tricky, and is no longer supported.  The
shared libraries are now called "extensions".  You should now write a
small Scheme file that calls `load-extension' to load the shared
library and initialize it explicitely.


Here is more about why "this has been found to be to tricky".  It is
about the way it was done, not about why it can't possible at all.

While this support was still present, modules could be either
implemented by Scheme source files, or by shared libraries compiled
from C.  These two forms are two very different things: one is
platform independent and installed somewhere in 'prefix', the other is
platform dependent and is installed in 'exec-prefix'.  However, Guile
had no platform dependent locations in its default search path.

Moreover, the search algorithm required shared libraries that were to
be autoloaded as modules to reside not in the usual library
directories (like /usr/local/lib), but in Guile search path as
.../foo/libbar.so for example for module (foo bar).  This will not
really work for shared libraries that are also to be used from C code.
Guile usually provides a C API for its features that are written in C.
This should be encouraged for extensions as well.  However, the Unix
shared library does not deal well with shared libraries that don't
come from standard locations or are referenced by multiple names
(symlinks).

Additionally, module boundaries are not necessarily language
boundaries.  That is, a module can be a mix of Scheme and C (and one
file might want to provide more than one module).  Therefore, we need
a good way to load shared libraries independently from modules anyway.

Restricting module system operations and autoloading to Scheme code
only provided an immediate and significant simplification, without
much hassle to the user.  The simplified setup should also be easier
to understand.

> More generally, looking back through mailing list history, it's
> actually astonishing how much support for various stuff that Guile
> has _lost_ along the way.

I'd say we didn't lose it, we freed us from it.

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