Seidr

In Old Norse, seiðr was a type of sorcery practiced in Norse society during the Late Scandinavian Iron Age. The practice of seiðr is believed to be a form of magic relating to both the telling and shaping of the future.

A collection of general purpose Sum Types and monads built on top of hojberg/sums-up

Maybe

import { Just, Nothing } from 'seidr';

Just("Loki").caseOf({
  Nothing: () => "N/A",
  Just: name => `Hello ${name}`,
}); // => "Hello Loki"

Nothing().caseOf({
  Nothing: () => "N/A",
  Just: name => `Hello ${name}`,
}); // => "N/A"

// Map doesn't run on Nothing
Just("Loki").map(name => name.toUpperCase()); // => Just("LOKI")
Nothing().map(name => name.toUpperCase()); // => Nothing()


// flatMap unnests a layer when the mapper returns a Maybe
Just("Loki").flatMap(name => Just(name.toUpperCase())); // => Just("LOKI")
Nothing().flatMap(name => Just(name.toUpperCase())); // => Nothing()

Result

import { Ok, Err } from 'seidr';

Ok("Loki").caseOf({
  Err: err => err,
  Ok: name => `Hello ${name}`,
}); // => "Hello Loki"

Err("oops").caseOf({
  Err: err => err
  Just: (name) => `Hello ${name}`,
}); // => "oops"

// Map doesn't run on Err
Ok("Loki").map(name => name.toUpperCase()); // => Just("LOKI")
Err("oops").map(name => name.toUpperCase()); // => Err("oops")

// flatMap unnests a layer when the mapper returns a Result
Ok("Loki").flatMap(name => Ok(name.toUpperCase())); // => Ok("LOKI")
Err("oops").flatMap(name => Ok(name.toUpperCase())); // => Err("oops")

// Result and Maybe are not isomorphic as "oops" is lost when converting Err to Nothing
Ok("Loki").toMaybe(); // => Just("Loki")
Err("oops").toMaybe(); // => Nothing()

AsyncResult

An AsyncResult<L, R> is a hybrid of a Result<L, R> and a Promise<R>, with the type safety and convenience methods of the former, but the ability to capture a not-necessarily-evaluated-yet value of the latter.

An AsyncResult<L, R> can be constructed from a Promise<R> with an appropriate function to convert the promise's unknown error value to an L.

There are also AsyncOk and AsyncErr functions available as shorthand, though in practice these are less frequently useful.

import { AsyncResult, AsyncOk, AsyncErr } from 'seidr';

////////////////////////////////////
// Constructing an AsyncResult

const myPromise: Promise<{ name: string }> = /* ... */;

AsyncResult.fromPromise(err => new Error(String(err)), myPromise);
// => AsyncResult<Error, { name: string }>


////////////////////////////////////
// Using an AsyncResult

// `caseOf` always returns a promise
AsyncOk("Loki").caseOf({
  Err: err => err,
  Ok: name => `Hello ${name}`,
}); // => Promise { <resolved>: "Hello Loki" }

AsyncErr("oops").caseOf({
  Err: err => err,
  Ok: (name) => `Hello ${name}`,
}); // => Promise { <resolved>: "oops" }

// `map` doesn't run on Err
AsyncOk("Loki").map(name => name.toUpperCase());  // => AsyncOk("LOKI")
AsyncErr("oops").map(name => name.toUpperCase()); // => AsyncErr("oops")

// `flatMap` unnests a layer when the mapper returns a Result, AsyncResult or Promise<Result>
AsyncOk("Loki").flatMap(name => Ok(name.toUpperCase()));       // => AsyncOk("LOKI")
AsyncOk("Loki").flatMap(name => AsyncOk(name.toUpperCase()));  // => AsyncOk("LOKI")
AsyncOk("Loki").flatMap(async name => Ok(name.toUpperCase())); // => AsyncOk("LOKI")
AsyncErr("oops").flatMap(name => Ok(name.toUpperCase()));      // => AsyncErr("oops")

// `toPromise` returns promise that resolves on Ok or rejects on Err
// A `Promise<R>` and an `AsyncResult<L, R>` are isomorphic at runtime,
// as the error value isn't lost; only the type information is.
AsyncOk("Loki").toPromise();  // Promise { <resolved>: "Loki" }
AsyncErr("oops").toPromise(); // Promise { <rejected>: "oops" }

Effect

import { Effect } from 'seidr';

const fooEff = Effect(() => "foo") // Effect("foo")

fooEff.unsafePerform() // => 'foo'

// Map doesn't run on Err
fooEff.map(val => val.toUpperCase()); // => Effect("FOO")

// flatMap unnests a layer when the mapper returns an Effect
fooEffect.flatMap(val => Effect(() => val + 'bar'); // => Effect("foobar")

RemoteData

A sums-up implementation of the original pattern described by Kris Jenkins: http://blog.jenkster.com/2016/06/how-elm-slays-a-ui-antipattern.html

This is a helpful sum type for modelling data being fetched from the network.

type RemoteData<E, A> = NotAsked | Loading | Failure<E> | Success<A>

import { RemoteData, NotAsked, Loading, Failure, Success } from 'seidr';

NotAsked().caseOf({
  NotAsked: () => "not asked",
  _: () => "everything else",
}); // => "not asked"

Loading().caseOf({
  Loading: () => "loading",
  _: () => "everything else",
}); // => "loading"

Failure("oops").caseOf({
  Failure: err => err
  _: () => "everything else",
}); // => "oops"

Success("Yay").caseOf({
  Success: data => data
  _: () => "everything else",
}); // => "yay"

// Map only runs on Success
Succecss("Loki").map(name => name.toUpperCase()); // => Just("LOKI")
Failure("oops").map(name => name.toUpperCase()); // => Failure("oops")
Loading().map(name => name.toUpperCase()); // => Loading()
NotAsked().map(name => name.toUpperCase()); // => NotAsked()

// flatMap unnests a layer when the mapper returns a Result when run on Success
Succecss("Loki").map(name => Success(name.toUpperCase())); // => Just("LOKI")
Failure("oops").map(name => Success(name.toUpperCase())); // => Failure("oops")
Loading().map(name => Success(name.toUpperCase())); // => Loading()
NotAsked().map(name => Success(name.toUpperCase())); // => NotAsked()