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:
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
foldlapplies it for us. So we can use
flip (-)instead of writing out the whole lambda
/x y -> y - x.
Additionally, it can be useful to use
flipto partially apply a function to its second argument. For example, we could use
flipto 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
flipin 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
flipdirectly is usually easier to read, but that’s a matter of taste.