#14782: typeclass polymorphism defeats bang patterns -------------------------------------+------------------------------------- Reporter: dmwit | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I think the following program should crash, but it doesn't: {{{ {-# LANGUAGE BangPatterns #-} main = let n :: Num a => a; !n = undefined in return () }}} Interestingly, my attempt to translate this out of BangPatterns into standard Haskell according to the translation on the wiki page led to a compile-time error: {{{ main = let n :: Num a => a; x@n = undefined in x `seq` return () }}} Produces the error: {{{ test.hs:1:12: error: • Overloaded signature conflicts with monomorphism restriction n :: forall a. Num a => a • In the expression: let n :: Num a => a x@n = undefined in x `seq` return () In an equation for ‘main’: main = let n :: Num a => a x@n = undefined in x `seq` return () | 1 | main = let n :: Num a => a; x@n = undefined in x `seq` return () | ^^^^^^^^^^^^^^^ }}} Even giving `x` its own explicitly polymorphic type signature -- what I thought was the standard way to prevent the monomorphism restriction from being a problem -- doesn't help. I'm uncertain what to make of that, but it certainly seems related: earlier versions of the compiler both do not give that error and do produce a crashing program, as I expected. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14782 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler