A collection of type-inference refactorings.
This patch does a raft of useful tidy-ups in the type checker. I've been meaning to do this for some time, and finally made time to do it en route to ICFP. 1. Modify TcType.ExpType to make a distinct data type, InferResult for the Infer case, and consequential refactoring. 2. Define a new function TcUnify.fillInferResult, to fill in an InferResult. It uses TcMType.promoteTcType to promote the type to the level of the InferResult. See TcMType Note [Promoting a type] This refactoring is in preparation for an improvement to typechecking pattern bindings, coming next. I flirted with an elaborate scheme to give better higher rank inference, but it was just too complicated. See TcMType Note [Promotion and higher rank types] 3. Add to InferResult a new field ir_inst :: Bool to say whether or not the type used to fill in the InferResult should be deeply instantiated. See TcUnify Note [Deep instantiation of InferResult]. 4. Add a TcLevel to SkolemTvs. This will be useful generally - it's a fast way to see if the type variable escapes when floating (not used yet) - it provides a good consistency check when updating a unification variable (TcMType.writeMetaTyVarRef, the level_check_ok check) I originally had another reason (related to the flirting in (2), but I left it in because it seems like a step in the right direction. 5. Reduce and simplify the plethora of uExpType, tcSubType and related functions in TcUnify. It was such an opaque mess and it's still not great, but it's better. 6. Simplify the uo_expected field of TypeEqOrigin. Richard had generatlised it to a ExpType, but it was almost always a Check type. Now it's back to being a plain TcType which is much, much easier. 7. Improve error messages by refraining from skolemisation when it's clear that there's an error: see TcUnify Note [Don't skolemise unnecessarily] 8. Type.isPiTy and isForAllTy seem to be missing a coreView check, so I added it 9. Kill off tcs_used_tcvs. Its purpose is to track the givens used by wanted constraints. For dictionaries etc we do that via the free vars of the /bindings/ in the implication constraint ic_binds. But for coercions we just do update-in-place in the type, rather than generating a binding. So we need something analogous to bindings, to track what coercions we have added. That was the purpose of tcs_used_tcvs. But it only worked for a /single/ iteration, whereas we may have multiple iterations of solving an implication. Look at (the old) 'setImplicationStatus'. If the constraint is unsolved, it just drops the used_tvs on the floor. If it becomes solved next time round, we'll pick up coercions used in that round, but ignore ones used in the first round. There was an outright bug. Result = (potentialy) bogus unused-constraint errors. Constructing a case where this actually happens seems quite trick so I did not do so. Solution: expand EvBindsVar to include the (free vars of the) coercions, so that the coercions are tracked in essentially the same way as the bindings. This turned out to be much simpler. Less code, more correct. 10. Make the ic_binds field in an implication have type ic_binds :: EvBindsVar instead of (as previously) ic_binds :: Maybe EvBindsVar This is notably simpler, and faster to use -- less testing of the Maybe. But in the occaional situation where we don't have anywhere to put the bindings, the belt-and-braces error check is lost. So I put it back as an ASSERT in 'setImplicationStatus' (see the use of 'termEvidenceAllowed') All these changes led to quite bit of error message wibbling