It’s useful as an argument to higher order functions (functions which take functions as arguments), where you want some particular value left unchanged.
Example 1: Leave a value alone if it is in a Just, otherwise, return a default of 7.
Prelude Data.Maybe> :t maybe
maybe :: b -> (a -> b) -> Maybe a -> b
Prelude Data.Maybe> maybe 7 id (Just 2)
2
Example 2: building up a function via a fold:
Prelude Data.Maybe> :t foldr (.) id [(+2), (*7)]
:: (Num a) => a -> a
Prelude Data.Maybe> let f = foldr (.) id [(+2), (*7)]
Prelude Data.Maybe> f 7
51
We built a new function f by folding a list of functions together with (.), using id as the base case.
Example 3: the base case for functions as monoids (simplified).
instance Monoid (a -> a) where
mempty = id
f `mappend` g = (f . g)
Similar to our example with fold, functions can be treated as concatenable values, with id serving for the empty case, and (.) as append.
Example 4: a trivial hash function.
Data.HashTable> h <- new (==) id :: IO (HashTable Data.Int.Int32 Int)
Data.HashTable> insert h 7 2
Data.HashTable> Data.HashTable.lookup h 7
Just 2
Hashtables require a hashing function. But what if your key is already hashed? Then pass the id function, to fill in as your hashing method, with zero performance overhead.