[mercury-users] Higher-order insts

[David Overton]

On Wed, May 19, 2004 at 02:26:21PM +1000, Peter Hawkins wrote:
> Hi...
> 
> Is there a way of storing a closure including higher-order insts in a data 
> structure and getting it back again? I'm trying to do this:
> 
> :- type something.
> 
> :- inst mypred = (pred(in, out) is semidet).
> :- type mypred = (pred(something, set(something))).
> :- mode mypredin == (mypred >> mypred).
> :- mode mypredout == (free >> mypred).
> 
> :- type athing ---> a(
> 	moo :: mypred
> ).
> 
> :- func new_athing(mypred) = athing.
> :- mode new_athing(mypredin) = out.
> new_athing(Moo) = a(Moo).
> 
> :- func get_moo(athing) = moo.
> :- mode get_moo(in) = mypredout.
> get_moo(AThing) = AThing ^ Moo.
> 
> But it fails with an error like this:
> things.m:098: In clause for `get_moo(in) = (things.mypredout)'
> :
> things.m:098:   mode error: argument 2 did not get sufficiently instantiated.
> things.m:098:   Final instantiatedness of `HeadVar__2' was `ground',
> things.m:098:   expected final instantiatedness was `(pred((ground -> ground), 
> (free -> ground)) is semidet)'.
> 
> Is what I am trying to do possible?

The mode declaration for `get_moo/1' needs to contain the higher-order
inst information.  E.g.:

:- inst athing ---> a(mypred).
:- mode get_moo(in(athing)) = mypredout.

You probably also want

:- mode new_athing(mypredin) = out(athing).

David
-- 
David Overton
WWW: http://www.overtons.id.au/
Mobile Phone (UK): +44 7799 344 322
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