[GHC] #9830: Standalone-derived Show instance for type constructor has different precedence if orphan instance

23 Nov 2014

      #9830: Standalone-derived Show instance for type constructor has different
precedence if orphan instance
-------------------------------------+-------------------------------------
       Reporter:  RyanGlScott        |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:  Incorrect
     Blocked By:                     |  result at runtime
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 If you are using {{{StandaloneDeriving}}} to derive a {{{Show}}} instance
 for a data type with an infix constructor, its precedence will be
 different depending on which module the {{{deriving instance ...}}}
 declaration is in. For example, with this code:

 {{{#!hs
 -- InfixShow.hs
 {-# LANGUAGE StandaloneDeriving #-}
 module InfixShow where

 infixr 6 :?:
 data ADT a b = a :?: b deriving (Eq, Ord, Read)

 deriving instance (Show a, Show b) => Show (ADT a b)
 }}}

 {{{#!hs
 -- Main.hs
 module Main where

 import InfixShow

 main :: IO ()
 main = do
     putStrLn $ "Prec 6:  " ++ showsPrec 6  ("test" :?: "show") ""
     putStrLn $ "Prec 7:  " ++ showsPrec 7  ("test" :?: "show") ""
     putStrLn $ "Prec 9:  " ++ showsPrec 9  ("test" :?: "show") ""
     putStrLn $ "Prec 10: " ++ showsPrec 10 ("test" :?: "show") ""
 }}}

 Calling {{{runhaskell Main.hs}}} will produce this output, as expected:

 {{{
 Prec 6:  "test" :?: "show"
 Prec 7:  ("test" :?: "show")
 Prec 9:  ("test" :?: "show")
 Prec 10: ("test" :?: "show")
 }}}

 However, if the code is changed so that the {{{deriving instance ...}}}
 declaration is in {{{Main.hs}}} instead:

 {{{#!hs
 -- InfixShow.hs
 module InfixShow where

 infixr 6 :?:
 data ADT a b = a :?: b deriving (Eq, Ord, Read)
 }}}

 {{{#!hs
 -- Main.hs
 {-# LANGUAGE StandaloneDeriving #-}
 module Main where

 import InfixShow

 deriving instance (Show a, Show b) => Show (ADT a b)

 main :: IO ()
 main = do
     putStrLn $ "Prec 6:  " ++ showsPrec 6  ("test" :?: "show") ""
     putStrLn $ "Prec 7:  " ++ showsPrec 7  ("test" :?: "show") ""
     putStrLn $ "Prec 9:  " ++ showsPrec 9  ("test" :?: "show") ""
     putStrLn $ "Prec 10: " ++ showsPrec 10 ("test" :?: "show") ""
 }}}

 Then the output of {{{runhaskell Main.hs}}} is different:

 {{{
 Prec 6:  "test" :?: "show"
 Prec 7:  "test" :?: "show"
 Prec 9:  "test" :?: "show"
 Prec 10: ("test" :?: "show")
 }}}

 This seems to indicate that {{{:?:}}} has the default application
 precedence (9) instead of the precedence defined in {{{InfixShow}}} (6).

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9830
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