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Standalone-derived Show instance for type constructor has different precedence if orphan instance

    • Closed Issue created by Ryan Scott @RyanGlScott

    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:

    -- 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)
    -- 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:

    -- InfixShow.hs
module InfixShow where

infixr 6 :?:
data ADT a b = a :?: b deriving (Eq, Ord, Read)
    -- 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 maximum operator precedence (9) instead of the precedence defined in InfixShow (6).

    Edited
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