#9323: Confusing type error behaviour -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Differential Revisions: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: -------------------------------------+------------------------------------- Compile this example with GHC 7.8.3. {{{ module Foo where broken :: [Int] broken = () ambiguous :: a -> String ambiguous _ = show 0 }}} You get {{{ Foo.hs:4:10: Couldn't match expected type ‘[Int]’ with actual type ‘()’ In the expression: () In an equation for ‘broken’: broken = () Foo.hs:7:15: No instance for (Show a0) arising from a use of ‘show’ The type variable ‘a0’ is ambiguous }}} (and a similar ambiguous `(Num a0)` error). '''But if you comment out `broken`, the program compiles.''', using the defaulting rules to choose `a0` = `Integer`. This is obviously wrong. Reported by Evan Laforge. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9323 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler