#15125: Typeclass instance selection depends on the optimisation level -------------------------------------+------------------------------------- Reporter: nicuveo | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by nicuveo: Old description:
(See the attached files for a minimal case.)
A file A defines a typeclass, and gives an incoherent instance for all types a, and exports a function relying on said typeclass. A file B defines some data types, makes them specific instances of that class. Which instance ends up being picked depends on the optimisation level they're compiled with.
//A.hs// {{{#!hs class A a where someValue :: a -> Maybe Int
instance {-# INCOHERENT #-} A a where someValue = const Nothing
getInt :: A a => a -> Int getInt x = fromMaybe 0 $ someValue x }}}
//B.hs// {{{#!hs data B = B Int
instance A B where someValue (B x) = Just x
getBInt :: Int getBInt = getInt $ B 42 }}}
//Main.hs// {{{#!hs main = print getBInt }}}
**Upon compiling with -O0, this prints 42; upon compiling with -O2, this print 0.**
(Interestingly, if the redundant class constraint is removed from getInt, then it always prints 0.)
New description: (See the attached files for a minimal case.) A file A defines a typeclass, and gives an incoherent instance for all types a, and exports a function relying on said typeclass. A file B defines some data types, makes them specific instances of that class, and uses the function defined in A. Which instance ends up being picked for B depends on the optimisation level those files are compiled with. //A.hs// {{{#!hs class A a where someValue :: a -> Maybe Int instance {-# INCOHERENT #-} A a where someValue = const Nothing getInt :: A a => a -> Int getInt x = fromMaybe 0 $ someValue x }}} //B.hs// {{{#!hs data B = B Int instance A B where someValue (B x) = Just x getBInt :: Int getBInt = getInt $ B 42 }}} //Main.hs// {{{#!hs main = print getBInt }}} **Upon compiling with -O0, this prints 42; upon compiling with -O2, this prints 0.** (Interestingly, if the redundant class constraint is removed from getInt, then it always prints 0.) -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15125#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler