Haskell: Purpose of the flip Function?

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I am a bit surprised that this was not asked before. Maybe it is a stupid question.

I know that flip is changing the order of two arguments.


(-) 5 3 = 5 - 3 = 2  flip (-) 5 3 = 3 - 5 = -2 

But why would I need such a function? Why not just change the Inputs manually?

Why not just write:

(-) 3 5 = 3 - 5 = -2 


One is unlikely to ever use the flip function on a function that is immediately applied to two or more arguments, but flip can be useful in two situations:

  1. If the function is passed higher-order to a different function, one cannot simply reverse the arguments at the call site, since the call site is in another function! For example, these two expressions produce very different results:

    ghci> foldl (-) 0 [1, 2, 3, 4] -10 ghci> foldl (flip (-)) 0 [1, 2, 3, 4] 2 

    In this case, we cannot swap the arguments of (-) because we do not apply (-) directly; foldl applies it for us. So we can use flip (-) instead of writing out the whole lambda /x y -> y - x.

  2. Additionally, it can be useful to use flip to partially apply a function to its second argument. For example, we could use flip to write a function that builds an infinite list using a builder function that is provided the element’s index in the list:

    buildList :: (Integer -> a) -> [a] buildList = flip map [0..] 
    ghci> take 10 (buildList (/x -> x * x)) [0,1,4,9,16,25,36,49,64,81] 

    Perhaps more frequently, this is used when we want to partially apply the second argument of a function that will be used higher-order, like in the first example:

    ghci> map (flip map [1, 2, 3]) [(+ 1), (* 2)] [[2,3,4],[2,4,6]] 

    Sometimes, instead of using flip in a case like this, people will use infix syntax instead, since operator sections have the unique property that they can supply the first or second argument to a function. Therefore, writing (`f` x) is equivalent to writing flip f x. Personally, I think writing flip directly is usually easier to read, but that’s a matter of taste.


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