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

[Ralph Becket]

> From: Fergus Henderson [mailto:fjh at cs.mu.OZ.AU]
> 
> > 	:- 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 `my_node'
> > 	:- pred node_type(my_node::unused) is det.
> 
> > 	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) },
> 		...

Sure.  Can we ditch : as a module separator sometime soon and use
: as a type qualifier instead?  :: is just going to confuse the issue.

> 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.

I know I'm being a bit slow, but could you give an example?

Cheers,

Ralph
--------------------------------------------------------------------------
mercury-users mailing list
post:  mercury-users at cs.mu.oz.au
administrative address: owner-mercury-users at cs.mu.oz.au
unsubscribe: Address: mercury-users-request at cs.mu.oz.au Message: unsubscribe
subscribe:   Address: mercury-users-request at cs.mu.oz.au Message: subscribe
--------------------------------------------------------------------------