I think these are useful in general, and that they would make a good addition to `Data.Ord`. The names are inspired by `sortOn`. Definitions:

```

minOn :: Ord b => (a -> b) -> a -> a -> a

minOn f x y =

case comparing f x y of

LT -> x

_ -> y

maxOn :: Ord b => (a -> b) -> a -> a -> a

maxOn f x y =

case comparing f x y of

GT -> x

_ -> y

```

These don't need to be the exact definitions, but they convey the semantics I'm looking for.