在承诺中使用Ramda拒绝

Question

How can Ramda.reject() be used with Promises?

map() can work like this

const data = [ {id: 1}, {id: 2} ]
const userIsAdmin = (user) => isAdmin(user) // async database call
const result = await Promise.all(R.map(userIsAdmin, data))
console.log(result) // [true, false]

But what I really want is to get the user ids back, but with admins rejected. I've tried the following code and other variations using pipeP .

await Promise.all(R.reject(async (d) => await userIsAdmin(d))(data))

How can this be done?

An alternative is to refactor isAdmin() to accept an array and just wait for one promise to complete.

Answer 1

I'm not sure I follow entirely, but can you not simply use reject on the results of Promise.all ?

const result = await Promise.all(R.map(asyncFn, data))
console.log(result) //=> [true, false]
console.log(R.filter(R.identity, result)) //=> [true]

But perhaps there's some confusion. You do want to use reject for what it's meant to do, right? It is simply a variant of filter with the predicate reversed, ie the (curried) equivalent of (fn, list) => R.filter(R.complement(fn), list) . It has nothing to do with the reject function supplied to the callback given in the Promise constructor.

Update

With the updated question, I think the easiest approach is still to use reject on the results of the Promise.all call, but you will probably have to combine it with the original data. Something like:

const removeAdmins = pipe(zip, reject(head), pluck(1))
removeAdmins(result, data)

zip combines the two (equal-length) lists into pairs. Then calling head on the pair gives you back the boolean, so reject(head) filters out those that returned true from your Promise.all calls. Then pluck(1) returns the second element (the original object) from each remaining pair.

Would that do what you want?