Re: [Haskell-cafe] type class constraints headache

methods :: (Eq a) => [(String, a)] methods = [ ("method1", undefined ) , ("method2", undefined) ]

enumerateMethodNames :: [String] enumerateMethodNames = map fst methods

The above does not compile because the source does not have enough information for GHC to determine what actual types to use for "methods" since undefined can stand in as values for any type. The program will compile if you use actual values instead of undefined or supply an explicit type signature. for example: enumerateMethodNames = map fst (methods :: [(String, String)]) or methods :: [(String, SomeEqType)] Rahul