EqPred pretty prints as ~ and equalities without brackets

compiler/parser/Parser.y.pp

@@ -971,8 +971,13 @@ ctype	:: { LHsType RdrName }
 -- errors in ctype.  The basic problem is that
 --	(Eq a, Ord a)
 -- looks so much like a tuple type.  We can't tell until we find the =>
+--
+-- We have the t1 ~ t2 form here and in gentype, to permit an individual
+-- equational constraint without parenthesis.
 context :: { LHsContext RdrName }
-	: btype 			{% checkContext $1 }
+        : btype '~'      btype  	{% checkContext
+					     (LL $ HsPredTy (HsEqualP $1 $3)) }
+	| btype 			{% checkContext $1 }
 
 type :: { LHsType RdrName }
 	: ipvar '::' gentype		{ LL (HsPredTy (HsIParam (unLoc $1) $3)) }
@@ -983,7 +988,7 @@ gentype :: { LHsType RdrName }
         | btype qtyconop gentype        { LL $ HsOpTy $1 $2 $3 }
         | btype tyvarop  gentype  	{ LL $ HsOpTy $1 $2 $3 }
  	| btype '->'     ctype		{ LL $ HsFunTy $1 $3 }
-        | btype '~'      gentype  	{ LL $ HsPredTy (HsEqualP $1 $3) }
+        | btype '~'      btype  	{ LL $ HsPredTy (HsEqualP $1 $3) }
 
 btype :: { LHsType RdrName }
 	: btype atype			{ LL $ HsAppTy $1 $2 }

compiler/types/TypeRep.lhs

@@ -232,7 +232,7 @@ Predicates are represented inside GHC by PredType:
 data PredType 
   = ClassP Class [Type]		-- Class predicate
   | IParam (IPName Name) Type	-- Implicit parameter
-  | EqPred Type Type		-- Equality predicate (ty1 :=: ty2)
+  | EqPred Type Type		-- Equality predicate (ty1 ~ ty2)
 
 type ThetaType = [PredType]
 \end{code}
@@ -251,7 +251,7 @@ represented by evidence (a dictionary, for example, of type (predRepTy p).
 
 Note [Equality predicates]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
-	forall a b. (a :=: S b) => a -> b
+	forall a b. (a ~ S b) => a -> b
 could be represented by
 	ForAllTy a (ForAllTy b (FunTy (PredTy (EqPred a (S b))) ...))
 OR
@@ -395,7 +395,7 @@ isLiftedTypeKind (TyConApp tc []) = isLiftedTypeKindCon tc
 isLiftedTypeKind other            = False
 
 isCoercionKind :: Kind -> Bool
--- All coercions are of form (ty1 :=: ty2)
+-- All coercions are of form (ty1 ~ ty2)
 -- This function is here rather than in Coercion, 
 -- because it's used in a knot-tied way to enforce invariants in Var
 isCoercionKind (NoteTy _ k)         = isCoercionKind k
@@ -436,7 +436,7 @@ pprParendType ty = ppr_type TyConPrec ty
 pprPred :: PredType -> SDoc
 pprPred (ClassP cls tys) = pprClassPred cls tys
 pprPred (IParam ip ty)   = ppr ip <> dcolon <> pprType ty
-pprPred (EqPred ty1 ty2) = sep [ppr ty1, nest 2 (ptext SLIT(":=:")), ppr ty2]
+pprPred (EqPred ty1 ty2) = sep [ppr ty1, nest 2 (ptext SLIT("~")), ppr ty2]
 
 pprClassPred :: Class -> [Type] -> SDoc
 pprClassPred clas tys = parenSymOcc (getOccName clas) (ppr clas)