Question

I’m haskell beginner. I try to write function that’s will return penultimate list element.

I start so:

lastButOne :: [a] -> a
lastButOne list = if null list
           then 
                0
           else 
                length list

0 string i get error:

Couldn't match type `a' with `Int'
      `a' is a rigid type variable bound by
          the type signature for lastButOne :: [a] -> a

I understand it. But what can i do here? I don’t know list type. What can i put here?

Thank you.

Answer 1

0 is of type int, so it won’t work for all a. I would suggest instead to use Maybe a like so:

lastButOne :: [a] -> Maybe a
lastButOne [] = Nothing
lastButOne [x] = Nothing
lastButOne xs = Just $ list !! (length xs - 2)

Using guards:

lastButOne :: [a] -> Maybe a
lastButOne xs | length xs > 1 = Just $ list !! (length list - 2)
              | otherwise = Nothing

Using the MonadPlus instance of Maybe:

import Control.Monad (guard)

lastButOne :: [a] -> Maybe a
lastButOne list = guard (length list > 1) >> return (list !! (length list - 2))

In general, for any instance of MonadPlus:

import Control.Monad (guard)

lastButOne :: MonadPlus m => [a] -> m a
lastButOne list = guard (length list > 1) >> return (list !! (length list - 2))

Although somewhere you will have to specify to the compiler that you want the result as a Maybe. Usually this will be type inferred because it will be passed to a function expecting a Maybe.

Please keep in mind that this runs in O(n). You probably don’t want to be finding the penultimate element of a list… Why do you need to do this? At least consider using an array instead.