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Estimated hours taken: 240 Branches: main Implement functional dependencies for the typeclass system. The implementation has two major parts. First, some of the basic checks of constraints are relaxed. These used to occur in make_hlds but that functionality has now been moved to check_typeclass. We also add a range of new tests to ensure that the FDs are used correctly. Second, an "improvement" pass to context reduction is added. This looks for constraints which match certain rules, and when it finds them updates the current bindings. The general rule is that type variables become more instantiated, but only in a way which provably does not affect the satisfiability of the constraints. XXX The plan for this change is to put the context reduction into a new module check_hlds.typeclasses.m, but I have left the code in typecheck.m for the moment because the diff will be easier to review that way. Moving to the new module will also remove the problem of one particular function being implemented in both typecheck and hlds_data, which is flagged by an XXX in the code. XXX the check for consistency of instances is not yet complete. We check all visible instances, but not instances that are only present at link time. We could check these in a similar way to the check for overlapping instances (that is, by defining a symbol that will conflict and cause a link error if there are overlapping instances), but in the long run a better solution will be required. Producing this is left for a later change. compiler/check_typeclass.m: Check for ambiguities in typeclass constraints here, rather than make_hlds. We check by calculating the closure of the bound type variables under the induced functional dependencies. This pass is merged in with the already existing pass that checks the quantifiers on constrained type variables. Check instances for range-restrictedness and for consistency. When checking for cycles in the typeclass hierarchy, build up the set of ancestors of a class which have FDs on them. This set is used when searching for opportunities to apply improvement rules during type checking. compiler/hlds_data.m: Define hlds_class_fundeps and add it to to hlds_class_defn. Add a field to hlds_class_defn to store the ancestors which have functional dependencies. Define the type 'instance_id', which is just an integer. This is what is used in proofs to identify instances. In hlds_constraints and in constraint_ids, use the terms 'assumed' and 'unproven' rather than 'existential' and 'universal'. The latter are confusing to use since the treatment of constraints differs depending on whether the constraint is on the head or the body of a clause. Add a field to the hlds_constraints for redundant constraints. These are constraints that have either already been reduced or don't need to be reduced, which may contribute to improvement of types. Define some new predicates for initialising and updating the hlds_constraints. compiler/type_util.m: Accommodate the change to hlds_constraints. compiler/hlds_out.m: Output the functional dependencies. compiler/intermod.m: Reconstruct a functional dependency from the HLDS, for outputting. compiler/make_hlds.m: Convert functional dependencies from parse tree form and add them to the HLDS. Check that functional dependencies are identical in subsequent definitions of the same typeclass. Don't check for ambiguity here. That is now done in check_typeclass. compiler/mercury_to_mercury.m: Output functional dependencies in typeclass declarations. compiler/prog_data.m: Define prog_fundeps and add them to the parse tree. compiler/prog_io_typeclass.m: Parse functional dependencies on typeclass declarations. compiler/typecheck.m: Require the class_table to be passed to reduce_context_by_rule_application, since the functional dependencies are stored here. Also thread the bindings argument through, since the bindings may be improved by context reduction. Save the resulting bindings in the type_assign. Instead of passing a list of assumed constraints and threading the unproven constraints through context reduction, thread through a hlds_constraints structure. This contains more information about redundant constraints than just the two lists. Extend context reduction with two new passes. The first applies the "class" FD rule, which tries to find two constraints which are identical on the domain of some FD, and then unifies the range arguments. The pair of constraints are either both redundant constraints, or one redundant constraint and one assumed constraint. The second applies the "instance" FD rule, which for each constraint tries to find an instance which is more general on the domain arguments. It then binds the instance arguments and unifies the range arguments of the instance with those of the constraint. When calculating the head_type_params for a predicate, include all variables that occur in universal constraints, since these may not necessarily occur in the arguments. Rename some variables: use variable prefixes "Pred" and "Parent" for types that are from the callee or that have been renamed apart respectively. This follows the same naming scheme used in polymorphism. Remove the headtypes/0 type, and use head_type_params/0 throughout. Add a new kind of cons_error for using "new" on a constructor that is not existentially typed. We check for this situation in convert_cons_defn, and report it in report_cons_error. Pass a value to convert_cons_defn indicating whether the constraints should be flipped or not, and whether the context is a constructor that uses 'new'. We flip the constraints here rather than after the fact, since creating the constraints now requires some extra processing to be done, and we don't want to have to redo that processing. Add a constant function that specifies whether variable numbers should be displayed as part of the debugging output. This is currently set to 'yes' but the previous behaviour can be achieved by changing the value to 'no'. doc/reference_manual.texi: Document the new feature. NEWS: Announce the new feature. tests/*: New test cases. compler/*.m: Minor changes related to the above. compiler/error_util.m: Fix comment grammar. compiler/prog_type.m: Fix an incorrect comment.
14 lines
240 B
Mathematica
14 lines
240 B
Mathematica
:- module inconsistent_instances.
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:- interface.
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:- typeclass foo(A, B) <= (A -> B) where [].
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:- implementation.
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:- import_module list.
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% Inconsistent.
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:- instance foo(list(T), int) where [].
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:- instance foo(list(X), string) where [].
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