coalesceEither.js

const {
  Either, coalesce, compose,
  constant, identity, ifElse,
  isString, map
} = require('crocks')

const { Left, Right } = Either

// onlyStrings : a -> Either Number String
const onlyStrings =
  ifElse(isString, Right, constant(Left(0)))

// shout : String -> String
const shout =
  str => str.toUpperCase()

// flow : a -> Either _ String
const flow = compose(
  coalesce(constant('[default]'), identity),
  map(shout),
  onlyStrings
)

console.log(flow(3))        // Right "[default]"
console.log(flow('three'))  // Right "THREE"