either-monad.js

eitherMonad.js

const Right = x =>
({
 chain: f => f(x),
 map: f => Right(f(x)),
 fold: (f, g) => g(x),
 toString: () => `Right(${x})`
})

const Left = x =>
({
 chain: f => Left(x),
 map: f => Left(x),
 fold: (f, g) => f(x),
 toString: () => `Left(${x})`
})

const fromNullable = x =>
 x != null ? Right(x) : Left(null)