Idioms are oblivious, arrows are meticulous, monads are promiscuous

Abstract

We revisit the connection between three notions of computation: Moggi’s monads, Hughes’s arrows and McBride and Paterson’s idioms (also called applicative functors). We show that idioms are equivalent to arrows that satisfy the type isomorphism A;B ' 1;(A ! B) and that monads are equivalent to arrows that satisfy the type isomorphism A;B ' A ! (1;B). Further, idioms embed into arrows and arrows embed into monads.