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 #16263

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/simplStg/RepType.hs b/compiler/simplStg/RepType.hs
index 4d437d3b7c..d93d716b4b 100644
--- a/compiler/simplStg/RepType.hs
+++ b/compiler/simplStg/RepType.hs
@@ -108,7 +108,7 @@ countFunRepArgs :: Arity -> Type -> RepArity
 countFunRepArgs 0 _
   = 0
 countFunRepArgs n ty
-  | FunTy arg res <- unwrapType ty
+  | FunTy _ arg res <- unwrapType ty
   = length (typePrimRepArgs arg) + countFunRepArgs (n - 1) res
   | otherwise
   = pprPanic "countFunRepArgs: arity greater than type can handle" (ppr (n, ty, typePrimRep ty))
@@ -120,7 +120,7 @@ countConRepArgs dc = go (dataConRepArity dc) (dataConRepType dc)
     go 0 _
       = 0
     go n ty
-      | FunTy arg res <- unwrapType ty
+      | FunTy _ arg res <- unwrapType ty
       = length (typePrimRep arg) + go (n - 1) res
       | otherwise
       = pprPanic "countConRepArgs: arity greater than type can handle" (ppr (n, ty, typePrimRep ty))