how to write the identity function in haskell

I have a function with type like this:

functionX :: [String] -> ([Integer] -> [Integer])

It is kind of like a mapping function 开发者_JAVA技巧that maps a specific String to a function with type as so. Because I need to handle the call functionX [], which I think this call should return something called identity function, or whatever, how can I write it?

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id is predefined as the identity function in haskell. It has type id :: a -> a.

If you wanted, you could define your own easily:

myIdentityFunction :: a -> a
myIdentityFunction a = a