References to values and weak references.

[Simon Marlow]

> I have just come up against an application where I would like 
> the user to be 
> able (if required) to link in a file containing a set of 
> default options to the
> application.  So what I would IDEALLY like is
> (1) the ability for FFI to give me access to an external 
> value as well as an external
>     function, as in 
>        foreign import "default_arguments" defaultArguments :: Addr

We have this.  It's called "foreign label", but it's in an obscure part of
the documentation - you're not the first person to miss it :)

> (2) the ability for FFI to link in values as weak external 
> references.  (In other words
>     the reference doesn't have to be resolved and you can 
> find out if it has been.)
>     So assuming (1) has been implemented, I might want 
> something additionally like
>        foreign import weak "default_arguments" 
> getDefaultArguments :: IO (Maybe Addr)
> Now while I would LIKE these features, they aren't there, and 
> it won't cause me great
> distress to work around their absence.  But why not implement 
> them anyway?

Do you know how to use existing support for weak references in the
linker/assembler?  I believe GNU as/ld has them, but I can't find any
reference in the docs.

Alternatively we'd have to implement this using something like dlsym, which
only looks for references in shared libraries.

Cheers,
	Simon