Interface Either<L, R>

Represents a type that can be either of type L or R. Either

Example: Use Either to handle erros

import { Either, left, right } from '@ortense/functors'

const divide = (
  numerator: number,
  denominator: number,
): Either<Error, number> => {
  if (Number.isNaN(numerator)) {
    return left(new Error('Numerator is not a number.'))
  }
  if (Number.isNaN(denominator)) {
    return left(new Error('Denominator is not a number.'))
  }

  if (denominator === 0) {
    return left(new Error('Division by zero is not posible.'))
  }

  return right(numerator / denominator)
}
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Example: capture correct side value

const numerator = Number(document.querySelector('input#numerator').value)
const denominator = Number(document.querySelector('input#denominator').value)
const display = document.querySelector('div#display-result')
const onSuccess = (result: number) => {
 display.textContent = `${numerator} / ${denominator} = ${result}`
}
const onError = (error: Error) => {
  display.textContent = error.message
}

divide(numerator, denominator)
  .right(onSuccess)
  .left(onError)
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Example: type-safe error handler

app.post('/users', async (req, res) => {
  const result: Either<Error, User> = await createUser(req.body)
  result
    .right(user => ({ id: user.id, name: user.name }))
    .right(responseBody => res.status(201).json(responseBody))
    .left(error => ({ error: error.name, message: error.message }))
    .left(responseBody => res.status(400).json(responseBody))
})
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