[m-dev.] Existentially quantified data constructors

[Mark Brown]

On 15-Apr-2008, Ralph Becket <rafe at csse.unimelb.edu.au> wrote:
> Julien Fischer, Tuesday, 15 April 2008:
> > 
> > On Tue, 15 Apr 2008, Ralph Becket wrote:
> > 
> > >I've defined a type:
> > >
> > >:- type lp_fd_pair(LPSolver, FDSolver)
> > >   --->    ( some [LPVar, FDVar]
> > >             lp_fd_pair(LPVar, FDVar)
> > >             =>    (   flatzinc_solver(LPSolver, LPVar),
> > >                       flatzinc_solver(FDSolver, FDVar)
> > >                   )
> > >           ).
> > >
> > >and received a complaint:
> > >
> > >flatzinc_fdlp_solver.m:093: Error: type variables in class constraints 
> > >introduced with `=>' must be explicitly existentially quantified using 
> > >`some': some [_3, _4] lp_fd_pair(_3, _4) => (flatzinc_solver(_1, _3), 
> > >flatzinc_solver(_2, _4)).
> > >flatzinc_fdlp_solver.m:321: Error: some but not all arguments have modes: 
> > >lp_fd_propagator_impl_2(_1, _1, (list(lp_fd_pair) :: in), (is_solved :: 
> > >out), (pqueue :: di), (pqueue :: uo)).
> > >
> > >Why can't I refer to LPSolver and FDSolver in the type class constraints
> > >on lp_fd_pair?
> > 
> > Presumably you are looking for a more enlightening answer than: it's not
> > valid Mercury?
> 
> I did manage to infer that from the error message...

It surprises me how often that doesn't happen.  :-(

Cheers,
Mark.

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