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

2 files changed, 9 insertions, 9 deletions

diff --git a/compiler/hieFile/HieAst.hs b/compiler/hieFile/HieAst.hs
index 7fd217cda5..9ac65ce399 100644
--- a/compiler/hieFile/HieAst.hs
+++ b/compiler/hieFile/HieAst.hs
@@ -31,7 +31,7 @@ import Name                       ( Name, nameSrcSpan, setNameLoc )
 import NameEnv                    ( NameEnv, emptyNameEnv, extendNameEnv, lookupNameEnv )
 import SrcLoc
 import TcHsSyn                    ( hsLitType, hsPatType )
-import Type                       ( mkFunTys, Type )
+import Type                       ( mkVisFunTys, Type )
 import TysWiredIn                 ( mkListTy, mkSumTy )
 import Var                        ( Id, Var, setVarName, varName, varType )
 import TcRnTypes
@@ -488,7 +488,7 @@ instance HasType (LHsExpr GhcTc) where
       fallback = makeNode e' spn
 
       matchGroupType :: MatchGroupTc -> Type
-      matchGroupType (MatchGroupTc args res) = mkFunTys args res
+      matchGroupType (MatchGroupTc args res) = mkVisFunTys args res
 
       -- | Skip desugaring of these expressions for performance reasons.
       --
diff --git a/compiler/hieFile/HieUtils.hs b/compiler/hieFile/HieUtils.hs
index 5259ea1280..9231317bd0 100644
--- a/compiler/hieFile/HieUtils.hs
+++ b/compiler/hieFile/HieUtils.hs
@@ -63,7 +63,7 @@ resolveVisibility kind ty_args
       where
         ts' = go (extendTvSubst env tv t) res ts
 
-    go env (FunTy _ res) (t:ts) -- No type-class args in tycon apps
+    go env (FunTy { ft_res = res }) (t:ts) -- No type-class args in tycon apps
       = (True,t) : (go env res ts)
 
     go env (TyVarTy tv) ts
@@ -81,8 +81,8 @@ hieTypeToIface = foldType go
     go (HLitTy l) = IfaceLitTy l
     go (HForAllTy ((n,k),af) t) = let b = (occNameFS $ getOccName n, k)
                                   in IfaceForAllTy (Bndr (IfaceTvBndr b) af) t
-    go (HFunTy a b) = IfaceFunTy a b
-    go (HQualTy pred b) = IfaceDFunTy pred b
+    go (HFunTy a b)     = IfaceFunTy VisArg   a    b
+    go (HQualTy pred b) = IfaceFunTy InvisArg pred b
     go (HCastTy a) = a
     go HCoercionTy = IfaceTyVar "<coercion type>"
     go (HTyConApp a xs) = IfaceTyConApp a (hieToIfaceArgs xs)
@@ -158,12 +158,12 @@ getTypeIndex t
       k <- getTypeIndex (varType v)
       i <- getTypeIndex t
       return $ HForAllTy ((varName v,k),a) i
-    go (FunTy a b) = do
+    go (FunTy { ft_af = af, ft_arg = a, ft_res = b }) = do
       ai <- getTypeIndex a
       bi <- getTypeIndex b
-      return $ if isPredTy a
-                  then HQualTy ai bi
-                  else HFunTy ai bi
+      return $ case af of
+                 InvisArg -> HQualTy ai bi
+                 VisArg   -> HFunTy ai bi
     go (LitTy a) = return $ HLitTy $ toIfaceTyLit a
     go (CastTy t _) = do
       i <- getTypeIndex t