[mercury-users] Typeclass problem (bug, misunderstanding, oth er?)

[Fergus Henderson]

On 02-Aug-2000, Ralph Becket <rbeck at microsoft.com> wrote:
> In your reply to my other typeclass help request, you suggest adding
> a dummy parameter to the function in question:
> 
> 	:- func solve(CSP, Node) = CSP <= csp(CSP, Node).
> 	:- mode solve(in, unused) = out is det.
> 
> 	...
> 	main -->
> 		{ node_type(DummyNode) },
> 		{ Sol = solve(CSP, DummyNode) },
> 		...
> 
> 	% this predicate just constrains the type of its
> 	% argument to be `int'
> 	:- pred node_type(my_node::unused) is det.

BTW, `int' there is a typo -- to make the comment match the
code, it should be `my_node'.  Sorry 'bout that.

> 	node_type(_).
> 
> While this will work, it does seem to be something of a hack.  I
> wonder if there's some small addition we could make to the language
> that would handle this more gracefully?

One thing that would certainly help is syntax for explicit type
qualification.  Then you could write it as

	main -->
		{ Sol = solve(CSP, _::my_node) },
		...

without needing to define the `node_type' predicate.
This one has been on the list for a while, and will
probably actually happen sometime soon.

> E.g.
> 
> :- func solve(CSP) = CSP <= (some [Node] csp(CSP, Node)).
> 
> solve(CSP0) = CSP :-
> 	some [PoppedNodes, TopNode] (
> 		... code in which it would otherwise be
> 		... impossible to work out a type for
> 		... PoppedNodes and TopNode, but which
> 		... doesn't need to know
> 	).

That won't work in general, since the choice of the node type
could affect the semantics.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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