Operator section

A partially applied infix operator: (a op) means \x -> a op x, and (op b) means \x -> x op b, subject to Haskell's syntax rules. In particular, (f .) maps a function g to the function composition f . g. Composing such a section with a curried binary function composes at two argument levels: ((f .) . g) x y = f (g x y) on total inputs.^[ref]