Add AnonArgFlag to FunTy
The big payload of this patch is:
Add an AnonArgFlag to the FunTy constructor of Type, so that (FunTy VisArg t1 t2) means (t1 -> t2) (FunTy InvisArg t1 t2) means (t1 => t2)
The big payoff is that we have a simple, local test to make
when decomposing a type, leading to many fewer calls to
isPredTy. To me the code seems a lot tidier, and probably
more efficient (isPredTy has to take the kind of the type).
See Note [Function types] in TyCoRep.
There are lots of consequences
- I made FunTy into a record, so that it'll be easier when we add a linearity field, something that is coming down the road.
- Lots of code gets touched in a routine way, simply because it pattern matches on FunTy.
- I wanted to make a pattern synonym for (FunTy2 arg res), which picks out just the argument and result type from the record. But alas the pattern-match overlap checker has a heart attack, and either reports false positives, or takes too long. In the end I gave up on pattern synonyms.
There's some commented-out code in TyCoRep that shows what I wanted to do.
- Much more clarity about predicate types, constraint types and (in particular) equality constraints in kinds. See TyCoRep Note [Types for coercions, predicates, and evidence] and Note [Constraints in kinds].
This made me realise that we need an AnonArgFlag on AnonTCB in a TyConBinder, something that was really plain wrong before. See TyCon Note [AnonTCB InivsArg]
- When building function types we must know whether we need VisArg (mkVisFunTy) or InvisArg (mkInvisFunTy). This turned out to be pretty easy in practice.
- Pretty-printing of types, esp in IfaceType, gets tidier, because we were already recording the (->) vs (=>) distinction in an ad-hoc way. Death to IfaceFunTy.
- mkLamType needs to keep track of whether it is building (t1 -> t2) or (t1 => t2). See Type Note [mkLamType: dictionary arguments]
Other minor stuff
- Some tidy-up in validity checking involving constraints; Trac Trac #16263