Odtnhj

I'm still getting the hang of Functional Programming and am trying to figure out a bit of a curly issue with Monads. I have a situation where I have a future that makes an HTTP request and returns a list of values, for argument's sake. I then want to be able to check that a particular value exists in that list, I would have thought returning a Maybe monad here would be the win. But then if that value is present I want to be able to make another HTTP request based on that returned value but it should only run if that value exists. Here's an example that uses ramda-fantasy implementations of Future & Maybe https://codesandbox.io/s/2xvy3m1qmy

I'm quite possibly getting this whole thing wrong, so if you need more info, or need to change any of the above, go for it. Just to clarify this would be my workflow in pseudo code:

1. Make a request for all values


2. filter values and maybe find one that matches


3. if found, make a request to get the rest of that data

Here's an initial, commented attempt, as linked above:

import R from "ramda";
import Fantasy from "ramda-fantasy";
const Future = Fantasy.Future;
const Maybe = Fantasy.Maybe;

// make a fake HTTP request and return a list of values
// response :: Future Array String
const response = Future.of(['one', 'two', 'three'])

const maybeOrNothing = val => val ? Maybe.Just(val) : Maybe.Nothing()

// Maybe return our given value
// getVal :: String -> Maybe String
const getVal = input => response.map(R.find(R.equals(input))).map(maybeOrNothing)

// make another fake request
// let's pretend that this takes an ID and makes a fake ajax request to get teh data
// getValueData :: String -> Future
const getValueData = id => Future((reject, resolve) => 
 // fake HTTP request
 setTimeout(() => 
 resolve(
 id: id,
 foo: 'bar'
 )
 , 100)
)

// 'one' should then run getValueData
// something isn't right here, do I need to map then chain?
getVal('one')
.chain(getValueData)
.fork(console.error, console.log)

// 'five' isn't there, so shouldn't run getValueData
// something isn't right here as getValueData still runs
// map(R.chain) works to not run getValueData but then it causes issues later on
getVal('five')
.chain(getValueData)
.fork(console.error, console.log)

You must be looking for the sequence operations that could turn the Maybe<Future<…>> into a Future<Maybe<…>> which you can then join into your outer future.

With traverse:

getVal('one')
.chain(traverse(Future.of, getValueData))
.fork(console.error, console.log)

Managed to solve my own problem, partially because of Bergi's suggestion. Bergi's answer above was very close, but the implementation wasn't quite right. The issue was that I was trying to deal with a Future and then change it to a Maybe, so I dealt with it with the following:

import R from "ramda";
import Fantasy from "ramda-fantasy";
const Future = Fantasy.Future;
const Maybe = Fantasy.Maybe;

// make a fake HTTP request and return a list of values
// response :: Future Array String
const response = Future.of(['one', 'two', 'three'])

const maybeOrNothing = val => val ? Maybe.Just(val) : Maybe.Nothing()
const maybeToFuture = m => Maybe.isNothing(m) ? Future.reject() : Future.of(m.value)

// Maybe return our given value
// getVal :: String -> Maybe String
const getVal = input => response.map(R.find(R.equals(input))).map(maybeOrNothing)

// make another fake request
// let's pretend that this takes an ID and makes a fake ajax request to get teh data
// getValueData :: String -> Future
const getValueData = id => Future((reject, resolve) => 
 // fake HTTP request
 setTimeout(() => 
 resolve(
 id: id,
 foo: 'bar'
 )
 , 100)
)

// 'one' should then run getValueData
getVal('one')
 .chain(maybeToFuture)
 .chain(getValueData)
 .fork(console.error, console.log)

// five isn't there so getValueData doesn't run
getVal('five')
 .chain(maybeToFuture)
 .chain(getValueData)
 .fork(console.error, console.log)