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Software	en	Lambda expressions	Haskell

You can define a function like:

double :: Int -> Int
double x = x + x

Which can also be written as an anonymous function:

\x -> x + x

Here, the \ symbol represents the Greek letter lambda: λ. This is derived from lambda calculus.

Lambda expressions can be used to more explicitly state that a function is returned.

Consider:

const :: a -> b -> a
const x _ = x

This can be written using a lambda expression and added parenthesis in the type definition. This is more explicit in that a function is being returned.

const :: a -> (b -> a)
const x = \_ -> x

And as an anonymous function. Consider the difference between these similar functions that return a list of odd numbers:

odds :: Int -> [Int]
odds n = map f [0..n-1]
         where f x = x*2 + 1

odds :: Int -> [Int]
odds n = map (\x -> x*2 + 1) [0..n-1]

-- > odds 15
-- > [1,3,5,7,9,11,13,15,17,19,21,23,25,27,29]