PureEvalEither
Either is a functor used for safely implementing error-handling flows, consisting of a Right value and a Left value.
Here's an example demonstrating the use of the Either functor:
function parseJSON(str) {
try {
const result = JSON.parse(str);
return Right(result);
} catch (e) {
return Left(e.message);
}
}
parseJSON('{sfdas').map(JSON.stringify).fold(console.error, console.log);function parseJSON(str) {
try {
const result = JSON.parse(str);
return Right(result);
} catch (e) {
return Left(e.message);
}
}
parseJSON('{sfdas').map(JSON.stringify).fold(console.error, console.log);Right/Left
Right represents an acceptable value, and its value will be propagated through the Either chain and participate in mapping.Left represents an erroneous value, and its value will be passed through the Either chain in a read-only manner.
If an error occurs during the execution of Right, the Right value will turn into Left.
- Type
- Example
Right(1); // Right 1
Left(1); // Left 1Right(1); // Right 1
Left(1); // Left 1Either.map()
Maps the values inside Right or Left.
- Type
- Example
Right(1).map(add(1)); // Right 2
Left(1).map(add(1)); // Left 1Right(1).map(add(1)); // Right 2
Left(1).map(add(1)); // Left 1Either.fold()
Maps and folds the values inside Either.
Pass in a reject function and a resolve function. For Right, only resolve will be executed, and for Left, only reject will be executed.
- Type
- Example
Right(1).map(add(1)).fold(console.error, id); // 2
Left(1).map(add(1)).fold(console.error, id); // err: 1Right(1).map(add(1)).fold(console.error, id); // 2
Left(1).map(add(1)).fold(console.error, id); // err: 1Either.chain()
Adds a new Either task to the end of the Either chain.
- Type
- Example
const getUserById = (id) => {
//...do sth.
const user = { id: 1, name: 'Alice' };
return Right(user);
};
const getUserPosts = (user) => {
//...do sth.
const posts = [{ title: 'Post 1' }, { title: 'Post 2' }];
return Right(posts);
};
const getPostsByUserId = (id) => getUserById(id).chain(getUserPosts);
getPostsByUserId(1).fold(
(error) => console.error(error),
(posts) => console.log(posts)
);const getUserById = (id) => {
//...do sth.
const user = { id: 1, name: 'Alice' };
return Right(user);
};
const getUserPosts = (user) => {
//...do sth.
const posts = [{ title: 'Post 1' }, { title: 'Post 2' }];
return Right(posts);
};
const getPostsByUserId = (id) => getUserById(id).chain(getUserPosts);
getPostsByUserId(1).fold(
(error) => console.error(error),
(posts) => console.log(posts)
);