Implement overlapping type family instances.
An ordered, overlapping type family instance is introduced by 'type instance where', followed by equations. See the new section in the user manual (22.214.171.124) for details. The canonical example is Boolean equality at the type level: type family Equals (a :: k) (b :: k) :: Bool type instance where Equals a a = True Equals a b = False A branched family instance, such as this one, checks its equations in order and applies only the first the matches. As explained in the note [Instance checking within groups] in FamInstEnv.lhs, we must be careful not to simplify, say, (Equals Int b) to False, because b might later unify with Int. This commit includes all of the commits on the overlapping-tyfams branch. SPJ requested that I combine all my commits over the past several months into one monolithic commit. The following GHC repos are affected: ghc, testsuite, utils/haddock, libraries/template-haskell, and libraries/dph. Here are some details for the interested: - The definition of CoAxiom has been moved from TyCon.lhs to a new file CoAxiom.lhs. I made this decision because of the number of definitions necessary to support BranchList. - BranchList is a GADT whose type tracks whether it is a singleton list or not-necessarily-a-singleton-list. The reason I introduced this type is to increase static checking of places where GHC code assumes that a FamInst or CoAxiom is indeed a singleton. This assumption takes place roughly 10 times throughout the code. I was worried that a future change to GHC would invalidate the assumption, and GHC might subtly fail to do the right thing. By explicitly labeling CoAxioms and FamInsts as being Unbranched (singleton) or Branched (not-necessarily-singleton), we make this assumption explicit and checkable. Furthermore, to enforce the accuracy of this label, the list of branches of a CoAxiom or FamInst is stored using a BranchList, whose constructors constrain its type index appropriately. I think that the decision to use BranchList is probably the most controversial decision I made from a code design point of view. Although I provide conversions to/from ordinary lists, it is more efficient to use the brList... functions provided in CoAxiom than always to convert. The use of these functions does not wander far from the core CoAxiom/FamInst logic. BranchLists are motivated and explained in the note [Branched axioms] in CoAxiom.lhs. - The CoAxiom type has changed significantly. You can see the new type in CoAxiom.lhs. It uses a CoAxBranch type to track branches of the CoAxiom. Correspondingly various functions producing and consuming CoAxioms had to change, including the binary layout of interface files. - To get branched axioms to work correctly, it is important to have a notion of type "apartness": two types are apart if they cannot unify, and no substitution of variables can ever get them to unify, even after type family simplification. (This is different than the normal failure to unify because of the type family bit.) This notion in encoded in tcApartTys, in Unify.lhs. Because apartness is finer-grained than unification, the tcUnifyTys now calls tcApartTys. - CoreLinting axioms has been updated, both to reflect the new form of CoAxiom and to enforce the apartness rules of branch application. The formalization of the new rules is in docs/core-spec/core-spec.pdf. - The FamInst type (in types/FamInstEnv.lhs) has changed significantly, paralleling the changes to CoAxiom. Of course, this forced minor changes in many files. - There are several new Notes in FamInstEnv.lhs, including one discussing confluent overlap and why we're not doing it. - lookupFamInstEnv, lookupFamInstEnvConflicts, and lookup_fam_inst_env' (the function that actually does the work) have all been more-or-less completely rewritten. There is a Note [lookup_fam_inst_env' implementation] describing the implementation. One of the changes that affects other files is to change the type of matches from a pair of (FamInst, [Type]) to a new datatype (which now includes the index of the matching branch). This seemed a better design. - The TySynInstD constructor in Template Haskell was updated to use the new datatype TySynEqn. I also bumped the TH version number, requiring changes to DPH cabal files. (That's why the DPH repo has an overlapping-tyfams branch.) - As SPJ requested, I refactored some of the code in HsDecls: * splitting up TyDecl into SynDecl and DataDecl, correspondingly changing HsTyDefn to HsDataDefn (with only one constructor) * splitting FamInstD into TyFamInstD and DataFamInstD and splitting FamInstDecl into DataFamInstDecl and TyFamInstDecl * making the ClsInstD take a ClsInstDecl, for parallelism with InstDecl's other constructors * changing constructor TyFamily into FamDecl * creating a FamilyDecl type that stores the details for a family declaration; this is useful because FamilyDecls can appear in classes but other decls cannot * restricting the associated types and associated type defaults for a * class to be the new, more restrictive types * splitting cid_fam_insts into cid_tyfam_insts and cid_datafam_insts, according to the new types * perhaps one or two more that I'm overlooking None of these changes has far-reaching implications. - The user manual, section 126.96.36.199, is updated to describe the new type family instances.