On 2008.01.09 18:15:33 +0000, Fernando Rodriguez
Hi,
I have the following type and function:
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
Thanks!
Couple of things. Your head'' is trying to use pattern-matching, but guards (which are what you are actually using) require a Bool expression on the left hand side; guards just desugar into if-then-else clauses. In this case it'd look something like
head'' a = if Nil then Nothing else if Cons a _ then Just a else ???
This doesn't make sense. Nil is just Nil, it's not in Bool; the if can't do anything with that. Similarly, Cons a _ is just ConsCell and in the Show typeclass; it too is not Bool. If we turn it into pattern-matching:
head'' Nil = Nothing head'' Cons a _ = Just a
But this still doesn't work - one definition takes a single argument, and the other 3; Nil (1), vs [Cons, a, _] (3). So: head'' Nil = Nothing head'' (Cons a _) = Just a Parentheses force the Cons to be seen as a single argument. So I guess your final product would look like this, which is pretty much what you start out with.
data ConsCell a = Nil | Cons a (ConsCell a) deriving Show
head' :: ConsCell a -> Maybe a head' Nil = Nothing head' (Cons a _) = Just a
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