In short, you cannot. What if your main were: main = getArgs >>= print . first_h The compiler doesn't know the difference and so it needs a type. Simple fix:
main = print (first_h ([] :: [Char]))
-- Hal Daume III "Computer science is no more about computers | hdaume@isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume On Fri, 17 May 2002 tty@SAFe-mail.net wrote:
Hello, I am writing a function which returns an exception/error string should an unexpected parameter is passed in. Borrowing from the Maybe module I have the following: ------------------------------------------------------------ data Result a = Ex String | Value a deriving Show
-- Testing the Result a type first_h :: [a] -> Result a first_h [] = Ex "No list" first_h (x:xs) = Value x
-- Trying out the first_h main :: IO() main = print (first_h []) ------------------------------------------------------------
Which the compiler complains:
Ambiguous type variable(s) `a' in the constraint `Show a' arising from use of `print' at atype.hs:8 In the definition of `main': print (first_h [])
This is understandable since it is unable to unify the empty list with a concrete list of type 'a' i.e. there are an infinite types which would match. My question is how can I indicate to the compiler that it does not matter which type the empty list is since the return result is of a concreate type.
Thanks
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