const checkUsername = auth => auth.username
  ? Right.of(auth)
  : Left.of('No Username')
const checkPassword = auth => auth.password
  ? Right.of(auth)
  : Left.of('Password Empty')


const validateLogin = (auth, constraints) => {
  const checkMatch = auth => (auth.username === 'jihad' && auth.password === 'jihad123')
    ? Right.of(auth.username)
    : Left.of('Username && password mismatch')

  const formatError = e => `error: ${e}`
  const successfulMsg = username => `Welcome back, ${username}!`

  const composed = constraints.reduce((acc, cons) => acc.bind(cons), Either.of(auth))

  return composed
    .bind(checkMatch)
    .cata(formatError, successfulMsg)
}

const validateRegister = (auth, constraints) => { ... }

validateLogin(auth, [checkUsername, checkPassword, etc])()
validateRegister(auth, [checkUsername, checkPassword, etc])()