What does the flip function do?

Question

I am a newbie to purescript. This is the book Leanpub-purescript in which I'm learning. I can't understand what flip funtion is. Is that similar to swapping concepts?

> :type flip
forall a b c. (a -> b -> c) -> b -> a -> c

which means a value goes to b, then b to a, then c is itself??. I'm struck with this. Please explain flip concept and if the book I'm referring is not good, suggest some other materials

Answer 1

The flip function reverses the order of arguments of a two-argument function. Consider a simple subtract function:

subtract :: Int -> Int -> Int
subtract a b = a - b

subtract 4 3
-- 4 - 3 = 1

If flip is called on the subtract function, it changes which number is being subtracted from:

(flip subtract) 4 3
-- 3 - 4 = -1

It also works with functions of differing argument types:

showIntAndString :: Int -> String -> String
showIntAndString int string = (show int) <> string

showIntAndString 4 "asdf"
-- "4asdf"

(flip showIntAndString) "asdf" 4
-- "4asdf"

If it makes more sense to you, try looking at flip as a function which accepts a two-argument function as an argument and returns another two-argument function as a result:

flip :: forall a b c.
    (a -> b -> c) -- takes a function
    -> (b -> a -> c) -- returns a function with flipped arguments

One of the use cases for flip is when you want to partially apply a function, but the argument you want to partially apply is on the second place. You can then flip the original function, and partially apply the resulting function.