either

import { either } from 'patronum';
// or
import { either } from 'patronum/either';

Motivation

The method select one or other value based on condition. You can think about it as a ternary operator a ? b : c.

Formulae

$result = either($filter, then, other);

$result store contains value of then if $filter store contains true
$result store contains value of other if $filter store contains false

Arguments

$filter: Store<boolean> — The store contains condition how to select between then and other
then: Store<Then> | Then — First value can be a Store, and returned when $filter is true
other: Store<Other> | Other — Second value can be a Store too, returned only when $filter if false

Then — Generic type argument. Required only to distinguish this type from Other
Other — Generic type argument used for the alternative value when $filter is false

Returns

$result: Store<Then | Other> — Store contains one of the value depends on $filter value

Object form arguments

For some cases it is useful to add names for each argument.

$result = either({
  filter,
  then,
  other,
});

Types, descriptions, and usage the same as for positional arguments.

Example

const $showLatest = createStore(false);
const $latestTweets = createStore<Tweet[]>([]);
const $recommendedTweets = createStore<Tweet[]>([]);

export const $tweets = either($showLatest, $latestTweets, $recommendedTweets);

Try it

Select just one argument

Sometimes we want to write an inverted expression, like this imperative code:

if (!active) {
  return another;
}
return null;

We can write exactly null as is:

const $result = either($active, null, $another); // Store<null | Another>

We may want to use not for condition:

import { either, not } from 'patronum';

const $result = either(not($active), $another, null);

Alternative

import { createStore, combine } from 'effector';
const $showLatest = createStore(false);
const $latestTweets = createStore<Tweet[]>([]);
const $recommendedTweets = createStore<Tweet[]>([]);

export const $tweets = combine(
  $showLatest,
  $latestTweets,
  $recommendedTweets,
  (showLatest, latestTweets, recommendedTweets) =>
    showLatest ? latestTweets : recommendedTweets,
);