Filtering null values from an array of maybe types fails to produce an array of non-optional values

[Firehed]

Per the title: if you have an array of maybe types, and filter out null values from that array (e.g. .map(val => val != null), flow fails to detect the "unwrapping" of these values and keeps treating the array as one of maybes. As a result, totally valid and safe future operations emit errors during typechecking.

// @flow

// Problem concept
let maybes: Array<?number> = [1,2,3];
let actuals = maybes
  .filter(val => val != null) // Should now be Array<number>, but stays as Array<?number>
  .filter(val => val > 0);

// Discovered this way
type Story = {id: number};
let story_ids = [1, 2, 3];
let stories: Array<Story> = [{id: 2}, {id: 3}];
let example = story_ids 
  .map(id => stories.find(story => story.id === id))
  .filter(story => story != null)
  .filter(story => story.id > 2)

// Works
let works = story_ids
  .map(id => stories.find(story => story.id === id))
  .filter(story => story != null && story.id > 2)
  // all future operations require the null check as well

src/example.js:7
  7:   .filter(val => val > 0);
                      ^^^ null. This type cannot be compared to
  7:   .filter(val => val > 0);
                            ^ number

src/example.js:7
  7:   .filter(val => val > 0);
                      ^^^ undefined. This type cannot be compared to
  7:   .filter(val => val > 0);
                            ^ number

src/example.js:16
 16:   .filter(story => story.id > 2)
                              ^^ property `id`. Property cannot be accessed on possibly undefined value
 16:   .filter(story => story.id > 2)
                        ^^^^^ undefined

Poking around, it looks like this should work, but it's at best unclear how to get flow to pick up on this.

[gcanti]

let maybes: Array<?number> = [1,2,3]
let actuals = maybes
  .filter(Boolean)
  .filter(val => val > 0)

[vkurchatkin]

See: #1414

[eluchsinger]

@gcanti Does that work only for non falsy values?

[rszymula]

@Firehed @vkurchatkin can we reopen this? The solution accepted is too specific
like @eluchsinger is asking
@gcanti

This is only works for the specific case that the array is full of numbers

let maybes: Array<?number> = [1,2,3]
let actuals = maybes
.filter(Boolean)
.filter(val => val > 0)

what if I have
{
person: {
place: ?string
}
}

and I filter out all non null places?

or what if I want to do a mapping afterwards?

let actuals = maybes
.filter(Boolean)
.filter(val => val > 0)
.map(item => item + 5)

What now? How to fix flow in the general case?