Don't use unnecessary parens when printing types (Fix Trac 4107)

   f :: Eq a => a -> a
rather than
   f :: (Eq a) => a -> a

diff --git a/compiler/hsSyn/HsTypes.lhs b/compiler/hsSyn/HsTypes.lhs
index cb06a7f..806faf2 100644 (file)
--- a/compiler/hsSyn/HsTypes.lhs
+++ b/compiler/hsSyn/HsTypes.lhs
@@ -353,8 +353,16 @@ pprHsForAll exp tvs cxt
     forall_part = ptext (sLit "forall") <+> interppSP tvs <> dot
 
 pprHsContext :: (OutputableBndr name) => HsContext name -> SDoc
-pprHsContext []         = empty
-pprHsContext cxt = ppr_hs_context cxt <+> ptext (sLit "=>")
+pprHsContext []                = empty
+pprHsContext [L _ pred] 
+   | noParenHsPred pred = ppr pred <+> ptext (sLit "=>")
+pprHsContext cxt        = ppr_hs_context cxt <+> ptext (sLit "=>")
+
+noParenHsPred :: HsPred name -> Bool
+-- c.f. TypeRep.noParenPred
+noParenHsPred (HsClassP {}) = True
+noParenHsPred (HsEqualP {}) = True
+noParenHsPred (HsIParam {}) = False
 
 ppr_hs_context :: (OutputableBndr name) => HsContext name -> SDoc
 ppr_hs_context []  = empty

diff --git a/compiler/types/TypeRep.lhs b/compiler/types/TypeRep.lhs
index 819a71c..55beb28 100644 (file)
--- a/compiler/types/TypeRep.lhs
+++ b/compiler/types/TypeRep.lhs
@@ -452,9 +452,19 @@ pprTheta :: ThetaType -> SDoc
 pprTheta theta = parens (sep (punctuate comma (map pprPred theta)))
 
 pprThetaArrow :: ThetaType -> SDoc
-pprThetaArrow theta 
-  | null theta = empty
-  | otherwise  = parens (sep (punctuate comma (map pprPred theta))) <+> ptext (sLit "=>")
+pprThetaArrow []     = empty
+pprThetaArrow [pred] 
+  | noParenPred pred = pprPred pred <+> ptext (sLit "=>")
+pprThetaArrow preds  = parens (sep (punctuate comma (map pprPred preds))) <+> ptext (sLit "=>")
+
+noParenPred :: PredType -> Bool
+-- A predicate that can appear without parens before a "=>"
+--       C a => a -> a
+--       a~b => a -> b
+-- But   (?x::Int) => Int -> Int
+noParenPred (ClassP {}) = True
+noParenPred (EqPred {}) = True
+noParenPred (IParam {}) = False
 
 ------------------
 instance Outputable Type where