#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