Inferred type is not general enough

Hi, The following function generates a type error. headColumnwise :: [a] -> a headColumnwise line = [ head line | line <- p ] ERROR ch6ex1.hs:14 - Inferred type is not general enough *** Expression : headColumnwise *** Expected type : [a] -> a *** Inferred type : [[a]] -> [a] If I change the type of the function to [[a]] -> [a] it compiles fine. Isn't [a]->a the most general type here? Thanks