#11995: Can't infer type -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): This is correct behavior. Though I failed to find a mention of the fact in either the user manual or in the Haskell 2010 report, a function can use polymorphic recursion only when given a type signature. The [https://en.wikipedia.org/wiki/Polymorphic_recursion Wikipedia page] on polymorphic recursion may be helpful. When you stub out the `xs` with an underscore, GHC considers the type signature to be incomplete and it does type inference -- not type checking -- on the function. Your function is polymorphically recursive, and so this fails. Still remaining to do: * improve the error message to mutter about polymorphic recursion (not sure if this is at all possible) * find and/or create documentation to this effect. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11995#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler