[m-dev.] Existentially quantified data constructors

[Julien Fischer]

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?

Julien.
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