Sometimes, we run into situations where we end up with a Maybe within the context of another Maybe. Nested structures like this can get really confusing to work with. In this lesson, we’ll look at an example of this and see how chaincan help us out by flattening the nested Maybe
As we all know, inside an array return another array is nested array.
Inside observable return another observable get Observable of Observable.
The same inside Just return another Just, we get Just of Just:
const prop = require(‘crocks/Maybe/prop‘); const propPath = require(‘crocks/Maybe/propPath‘); /** * return Maybe type */ const getUser = id => new Promise((resolve, reject) => { const result = { status: 200, body: { id: 1, username: ‘tester‘, email: ‘test@gmail.com‘, address: { street: ‘111 E. West St‘, city: ‘Anywhere‘, state: ‘PA‘, postalCode: ‘19123-4567‘ } } } resolve(prop(‘body‘, result)); // return Just {} }); const getPostalCode = propPath([‘address‘, ‘postalCode‘]); getUser(1) .then(user => user.map(getPostalCode)) // map to Just {} --> Just Just ‘19123-4567‘ .then(console.log); // Just Just "19123-4567"
Inside getUser function we return a Just type object. Then from propPath, we get Just Just type object. That‘s why we need flatten it.
The way to solve the problem is using flat map which is called ‘chain‘ in Crocks.
getUser(1) .then(user => user.chain(getPostalCode).option("not available")) .then(console.log); // "19123-4567"
[Javascript Crocks] Flatten Nested Maybes with `chain`
原文：https://www.cnblogs.com/Answer1215/p/9026521.html