#9830: Standalone-derived Show instance for type constructor has different precedence if orphan instance -------------------------------------+------------------------------------- Reporter: | Owner: RyanGlScott | Status: new Type: bug | Milestone: Priority: normal | Version: 7.8.3 Component: Compiler | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Incorrect | Related Tickets: result at runtime | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Description changed by RyanGlScott: Old description:

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).

New description: 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 maximum operator precedence (9) instead of the precedence defined in {{{InfixShow}}} (6). -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9830#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#9830: Standalone-derived Show instance for type constructor has different precedence if orphan instance -------------------------------------+------------------------------------- Reporter: | Owner: RyanGlScott | Status: closed Type: bug | Milestone: Priority: normal | Version: 7.8.3 Component: Compiler | Keywords: Resolution: fixed | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Incorrect | Related Tickets: result at runtime | Test Case: | deriving/should_run/T9830 | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by simonpj): * status: new => closed * testcase: => deriving/should_run/T9830 * resolution: => fixed Comment: Thanks! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9830#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler