Idea: `imaximumOf` and `iminimumOf`

[ozkutuk]

Recently I needed a way to find the index of the maximum element in a list, and I was surprised that there was not an indexed counterpart to maximumOf. I came up with the following, which I thought could also be generally useful to have in optics:

newtype IxMin i a = IxMin { getIxMin :: Maybe (a, i) }
  deriving (Semigroup, Monoid) via Maybe (Min (a, i))

iminimumOf
  :: (Is k A_Fold, is `HasSingleIndex` i, Ord a, Ord i)
  => Optic' k is s a -> s -> Maybe (i, a)
iminimumOf o = fmap swap . getIxMin . ifoldMapOf o (curry (IxMin . Just . swap))

newtype IxMax i a = IxMax { getIxMax :: Maybe (a, i) }
  deriving (Semigroup, Monoid) via Maybe (Max (a, Down i))

imaximumOf
  :: (Is k A_Fold, is `HasSingleIndex` i, Ord a, Ord i)
  => Optic' k is s a -> s -> Maybe (i, a)
imaximumOf o = fmap swap . getIxMax . ifoldMapOf o (curry (IxMax . Just . swap))

I wrote them such that if there exists multiple maximum elements, the one with the "lesser" index is returned:

>>> iminimumOf ifolded "calamari"
Just (1,'a')

Is it desirable to have these in the library? If so, I can prepare a PR for them.

[arybczak]

I don't see why not 👍 Feel free to create a PR.

[phadej]

If you will write the patch, define IxMax (Maybe (i, a)) (and write manual instances) to avoid need to swap, and use #. and .# for when composing with getIxMax and IxMax. This will affect performance for the better.