Closure is a fundamental concept in functional programming and Haskell is a great language to learn about it.
In Haskell, a closure is a function that has access to some variables in its environment, even after those variables are no longer in scope. This means that the function 'closes over' those variables and can use them even if they are not explicitly passed as arguments to the function.
Here's an example of a closure in Haskell:
add :: Int -> Int -> Int
add x = \y -> x + y
In this example, the add function takes an Int as an argument and returns a lambda function. The lambda function is the actual closure that has access to the x variable (the first argument to the add function).
Here's how you might use this closure:
addFive :: Int -> Int
addFive = add 5
addFive 7 -- returns 12
In this example, we declare a new function addFive that is created by partially applying the add function with an x value of 5. This means that addFive is a new closure that has access to the x value of 5, which it can then add to any y value that is passed in.
When we call addFive with 7 as an argument, it returns 12 (i.e., 5 + 7).
So, you can use closures in Haskell to create new functions from existing ones by partially applying arguments, and those new functions can have access to variables from the original function.