The on combinator

λ> let outer f x y = fmap (\a -> fmap (f a) y) x
λ> let self f x = f x x    -- Like ⍨ from APL
λ> let on = self . outer

on is identical to the combinator provided by Data.Function.

λ> sortBy (compare `on` snd) [(1,"c"),(3,"a"),(4,"d")]
[(3,"a"),(1,"c"),(4,"d")]

      on ← { (⍵⍵ ⍺) ⍺⍺ (⍵⍵ ⍵) }  ⍝ Proposed Over (⍥) operator
      'ra' , on ⌽ 'yar'
array