Thinking about Promise.all, once there is a Promise in the list fails, the whole promise fails.
There would be good to have an ‘Alernative‘ operator, which just skip the failling one, and continue the work. ‘Alternative‘ works for Either data type;
const Right = (x) => ({ x, chain: (f) => f(x), map: (f) => Right(f(x)), concat: (o) => Right(x.concat(o.x)), fold: (f, g) => g(x), isRight: true, isLeft: false, toString: `Right(${x})`, }); const Left = (x) => ({ x, chain: (f) => Left(x), map: (f) => Left(x), concat: (o) => Left(x), fold: (f, g) => f(x), isRight: false, isLeft: true, toString: `Left(${x})`, }); const Alternative = (ex) => ({ ex, concat: (o) => Alternative(o.ex.isLeft ? ex : ex.concat(o.ex)), });
Run:
const res = Alternative(Right("hello"))
.concat(Alternative(Right("world")))
.concat(Alternative(Left("errrr")))
.concat(Alternative(Right("!!")));
console.log(res.ex.fold(identity, identity)); // helloworld!!
Simpify the code:
const List = (x) => ({ x, foldMap(type, _empty) { const empty = _empty ? _empty : type.empty(); if (!empty) throw new Error(`foldMap expect an empty as second value`); return x.map(type).reduce((acc, curr) => { return acc.concat(curr); }, empty); }, });
const res1 = List([ Right("hello"), Right("world"), Left("errrr"), Right("!!!!"), ]).foldMap(Alternative, Alternative(Right(""))); console.log(res1.ex.fold(identity, identity)); // helloworld!!!!
[Functional Programming] Alternative operator for Either
原文：https://www.cnblogs.com/Answer1215/p/13031395.html