[m-dev.] Existentially quantified data constructors

[Ralph Becket]

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...
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