Either Coroutines is an experimental Typescript library seeking to emulate the Either comprehensions and Effect Scopes provided by Kotlin ArrowKT but in the Typescript language.
It provides a minimal implementation of the Either monad including the following methods available on the monad instance:
- map
- flatMap
- mapLeft
- isRight
- isLeft
- unwrap
It also includes the following helper functions for dealing with Either monads:
- Either.left -> Creates a new Either as an instance of Left
- Either.right -> Creates a new Either as an instance of Right
- Either.runCatching -> wrap an unsafe function call and return the results as an Either
- Either.runSuspending -> Takes a generator function and allows you to perform comprehensions on any internal Either values. Will short-circuit execution if a Left is found.
Not currently available
import { Either } from './'
// Manually create Either values
const right = Either.right("Hello World")
const left = Either.left(new Error("My Error"))
// Wrap unsafe calls
function unsafe(input: number) {
if (input % 2 === 0) {
return input * 2
}
throw new Error("Not a multiple of two")
}
const resultA = Either.runCatching(() => unsafe(2))
console.assert(resultA.isRight())
const resultB = Either.runCatching(() => unsafe(5))
console.assert(resultB.isLeft())
// Perform comprehensions using the `yield x.bindValue()` syntax
function doubleEvenNumber(input: number): Either<Error, number> {
if (input % 2 === 0) {
return Either.right(input * 2)
}
return Either.left(new Error("Not a multiple of two"))
}
const doubleA = Either.runSuspending(function* () {
const toDouble = 2
const doubled = yield doubleEvenNumber(toDouble).bindValue()
console.log(`${toDouble} * 2 = ${doubled}`)
return doubled
})
console.assert(resultA.isRight())
const doubleB = Either.runSuspending(function* () {
const toDouble = 5
const doubled = yield doubleEvenNumber(toDouble).bindValue()
// This code will never run because doubleEvenNumber will return a left
// which will cause `runSuspending` to short-circuit execution
console.log()
console.log(`${toDouble} * 2 = ${doubled}`)
return doubled
})
console.assert(doubleB.isLeft())
Code contributions are not open at this time.
You are free to clone and fork the repo, or provide suggestions as issues, but I will not be accepting any PRs.