deleteBy :: (a -> b -> Bool) -> a -> [b] -> [b]

deleteFirstsBy :: (a -> b -> Bool) -> [a] -> [b] -> [b]

intersectBy :: (a -> b -> Bool) -> [a] -> [b] -> [a]

I approve these versions because:

1. It makes more sense that the "usual" `delete` and `intersect` functions are specialization of the "By" operations, not the vice versa.

2. Data.Map.Lazy module from containers package has these kind of "By" operations.

The previous discussion about this in Sep 2016 didn't make much conclusion, so let the discussion continue.