Going through the magnificent Haskell Programming from first principles, I’m currently reimplementing functions from standard libraries such as maximumBy and minimumBy.

Here are my current versions of them:

myMaximumBy :: (a -> a -> Ordering) -> [a] -> a -- maximumBy in Data.List also throws an error on empty lists myMaximumBy _ [] = error "empty list" myMaximumBy f (x:xs) = go xs x   where go [] curMax = curMax         go (h:t) curMax = go t $   if (f h curMax) == GT then h else curMax         -- Could be shorter, but less readable with bool from Data.Bool:         -- go (h:t) curMax = go t $   bool curMax h (f h curMax == GT)  myMinimumBy :: (a -> a -> Ordering) -> [a] -> a myMinimumBy f = myMaximumBy $   \x y -> negateOrdering $   f x y   where negateOrdering EQ = EQ         negateOrdering LT = GT         negateOrdering GT = LT 

I know they’re not total functions, but I’ll get to rewriting them with something like Maybe later on.

What I’m more curious about is whether it’s a problem to define myMinimumBy through myMaximumBy using negateOrdering; could this be a problem for some implementations of the ordering function f?

Additionaly to takes on the question in the last paragraph, I’m happy for any kind of feedback on how to make the code shorter, more readable, or more idiomatic.