Uses for Haskell id function

Question

Which are the uses for id function in Haskell?

Answer 1

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.