How to change the order of arguments?

Question

What if I want to change the order of arguments in a function?

There is flip:

flip :: (a -> b -> c) -> b -> a -> c
         


        

Answer 1

The best way in general is to just do it manually. Assume you have a function

f :: Arg1 -> Arg2 -> Arg3 -> Arg4 -> Res

and you would like

g :: Arg4 -> Arg1 -> Arg3 -> Arg2 -> Res

then you write

g x4 x1 x3 x2 = f x1 x2 x3 x4

If you need a particular permutation several times, then you can of course abstract from it, like flip does for the two-argument case:

myflip :: (a4 -> a1 -> a3 -> a2 -> r) -> a1 -> a2 -> a3 -> a4 -> r
myflip f x4 x1 x3 x2 = f x1 x2 x3 x4