[mercury-users] Another Mercury Modes Problem

[David Overton]

jn Fri, May 31, 2002 at 05:03:08PM +0900, Bob McKay wrote:
> 
> I think we're talking at cross purposes here. My overall aim is to
> write a system with at least three levels:
> .a top level, generic evolutionary algorithm
> .a mid level, with interchangeable instantiations of this to a
> 	particular evolutionary algorithm and data structure (eg genetic
> 	algorithm with bitstrings; evolutionary strategy with real
> 	representation; or a strange variant with clauses as the
> 	representation)
> .a bottom level, at which an individual problem - particular
> 	fitness function, representation  details such as dimensions -
> 	is defined.
> 
> So the outer level needs to define not only the required procedures,
> but parametric types and modes for them. They would be instantiated
> in the lower levels to define the actual types and modes. This seems
> straightforward for types, but not possible for modes for the reasons
> I previously mentioned (ie that you can't have parametric unique or
> bound modes). Or am I missing something here?

Parametric modes (i.e. modes containing inst variables) are supported.

E.g.

:- mode append(in(list_skel(I)), in(list_skel(I)), out(list_skel(I))).

defines a mode for append/3 where the elements of the input lists have
some inst `I' and the elements of the output list also have inst `I' after
the procedure succeeds.

There is an implicit constraint `I =< ground' in this example which says
that `I' is constrained to be a "sub-inst" of `ground'.  This includes
all higher-order insts and bound(...) insts where the arguments are
ground.  If you want `I' to be constrained to be a sub-inst of some other
inst, e.g.  `unique', you need to explicitly say this in the
declaration.  The syntax for this is similar to the syntax for type
class constraints.  E.g.

:- mode append(di(unique_list_skel(I)), di(unique_list_skel(I)),
	out(unique_list_skel(I))) <= I =< unique.

constrains `I' to be a unique inst.

Note that the second of these example only works with fairly recent ROTD
releases (since approx mid March).



David
-- 
David Overton      Computer Science and Software Engineering
PhD Student        The University of Melbourne   +61 3 8344 9159
Research Fellow    Monash University (Clayton)   +61 3 9905 5779
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