[m-dev.] Revisiting `any' and inst casts for solver types

[Fergus Henderson]

On 01-Jun-2004, Ralph Becket <rafe at cs.mu.OZ.AU> wrote:
> So here's yet another suggestion.  We start with some syntax like this:
> 
> :- solver type <<solver type name and params>>
> 	where	representation is <<representation type name and args>>,
> 		ground         is <<ground inst>>,
> 		any            is <<any inst>>,
> 		equality       is ...
> 
> This says that
> - <<solver type>> is implemented using <<representation type>>,
> - a `ground' <<solver type>> value has a corresponding <<ground inst>>
>   <<representation type>> value,
> - an `any' <<solver type>> value has a corresponding <<any inst>>
>   <<representation type>> value.

Sounds fine so far.

> The compiler will treat `church' as an opaque foreign type and will
> construct a predicate to handle conversion between `church' and its
> internal representation `ch':
> 
> :- impure pred 'church/0 representation'(church,   ch            ).
> :-        mode 'church/0 representation'(in(any),  out(any_ch)   ) is det.
> :-        mode 'church/0 representation'(out(any), in(any_ch)    ) is det.
> :-        mode 'church/0 representation'(in,       out(ground_ch)) is det.
> :-        mode 'church/0 representation'(out,      in(ground_ch) ) is det.

Note that if you give this predicate slightly different modes,
it need not be impure:

	:- pure pred 'church/0 representation'(church,   ch            ).
	:-      mode 'church/0 representation'(in(any),  out(any_ch)   ) is cc_multi.
	:-      mode 'church/0 representation'(out(any), in(any_ch)    ) is det.
	:-      mode 'church/0 representation'(in,       out(ground_ch)) is cc_multi.
	:-      mode 'church/0 representation'(out,      in(ground_ch) ) is det.

However, using `promise_pure' is in many cases a lot easier than using
`promise_only_solution', so until we have a better alternative to
`promise_only_solution', you may prefer to use the impure approach.

> Otherwise, everything is much the same as before.

Would you mind reposting your description of those parts?

-- 
Fergus Henderson                    |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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