I've been reviewing the library, and have come unstuck with the id function.
What's its purpose and can someone give me an example of its practical use.

It's purpose is simply to be the identity function. The type says exactly what it does.

Prelude> :t id
id :: a -> a

It's often useful in combination with higher-order functions.

Prelude> :t foldr (.) id
foldr (.) id :: [a -> a] -> a -> a

Here's a slightly complicated way to add a list of numbers:

Prelude> foldr (.) id (map (+) [1..5]) 0
15

Sean