有人可以解释下面的代码是如何工作的吗？

Question

I can't figure out how two recursions(myMaximumBy) work together, and I'm trying to draw diagram on paper but I'm stuck. For example, myMaximumBy compare [1, 5, 2, 4, 3]

myMaximumBy :: (a -> a -> Ordering) -> [a] -> a
myMaximumBy _ (x:[]) = x
myMaximumBy f (x:xs) = if (f x (myMaximumBy f xs)) == GT then x else (myMaximumBy f xs)

Answer 1

Basically you traverse the entire list until you hit a single element, x (first line). Since x is the only element, it must be the maxiumum.

Now you go backwards and check every element y against x: If y is greater than x (first case) then you continue with y as your maximum, otherwise you keep x (second case).

Instead of using your definition with the if-clause I will use maxBy to illustrate this:

maximumBy f [x] = x
maximumBy f (x:xs) = maxBy f x (maximumBy f xs)

maxBy f x y | f x y == GT = x
            | otherwise   = y

This definition is equivalent to yours.

Example:

maximumBy (comparing abs) [2,5,-3,1]
== maxBy (comparing abs) 2 (maxBy (comparing abs) 5 (maxBy (comparing abs) -3 (maximumBy (comparing abs) [1])))
== maxBy (comparing abs) 2 (maxBy (comparing abs) 5 (maxBy (comparing abs) -3 1))
== maxBy (comparing abs) 2 (maxBy (comparing abs) 5 -3)
== maxBy (comparing abs) 2 5
== 5