Fernando Rodriguez wrote:
data ConsCell a = Nil | Cons a (ConsCell a) deriving Show head' Nil = Nothing head' (Cons a _) = Just a
Works fine, however, what's wrong with the following function?
head'' | Nil = Nothing | Cons a _ = Just a
You cannot use | as a general shortcut in function definitions as you try here. The stroke is instead used to express different branches selected by boolean expressions: headIfNotZero Nil = Nothing headIfNotZero (Cons a) | a == 0 = Nothing | otherwise = Just a In this definition, (a == 0) and (otherwise) are boolean expressions. GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> otherwise True But in your definition of head'', Nil and (Cons a) are Patterns, not boolean expressions, so it doesnt work out. If you don't like to write out the name of the function you want to define over and over again, you can use an explicit case statement: head''' x = case x of Nil -> Nothing Cons a _ -> Just a This is sometimes easier to read, but in this case, I would use your head' definition. Tillmann