[Haskell-cafe] Trouble with function with two clauses

[Tillmann Rendel]

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