[GHC] #7645: Parens in an error message

[GHC]

#7645: Parens in an error message
-----------------------------+----------------------------------------------
Reporter:  monoidal          |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.1             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:  #7609           
-----------------------------+----------------------------------------------
 {{{
 {-# LANGUAGE TypeOperators #-}
 data (+) a b
 f :: ((+) a a, Maybe)
 f = undefined
 }}}

 gives

 {{{
 X.hs:3:16:
     Expecting one more argument to `Maybe'
     In the type signature for `f': f :: (+ a a, Maybe)
 }}}

 which should be `(+) a a`. I tried

 {{{
 #!diff
 diff --git a/compiler/hsSyn/HsTypes.lhs b/compiler/hsSyn/HsTypes.lhs
 index 74aa477..d0d9e1a 100644
 --- a/compiler/hsSyn/HsTypes.lhs
 +++ b/compiler/hsSyn/HsTypes.lhs
 @@ -614,7 +614,7 @@ ppr_mono_ty ctxt_prec (HsForAllTy exp tvs ctxt ty)
  ppr_mono_ty _    (HsBangTy b ty)     = ppr b <> ppr_mono_lty pREC_CON ty
  ppr_mono_ty _    (HsQuasiQuoteTy qq) = ppr qq
  ppr_mono_ty _    (HsRecTy flds)      = pprConDeclFields flds
 -ppr_mono_ty _    (HsTyVar name)      = ppr name
 +ppr_mono_ty _    (HsTyVar name)      = pprPrefixOcc name
  ppr_mono_ty prec (HsFunTy ty1 ty2)   = ppr_fun_ty prec ty1 ty2
  ppr_mono_ty _    (HsTupleTy con tys) = tupleParens std_con (interpp'SP
 tys)
    where std_con = case con of
 }}}

 but this causes the kind * to be printed as (*).

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7645>
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