Flow: "import type" silently ignored

[skovhus]

import type requires the file exporting types to have have // @flow comment in the file.

So currently flow doesn't silently ignores any requires of types from files without a @flow comment. This can take a long time to debug...

So if I have two files defs.js and main.js no error is catched by flow:

# defs.js
export type UserID = number;

# main.js
// @flow
import type { UserID } from './defs';
export function myFunc(): UserID {
    return '42';
}

Even importing something that is not exported doesn't fail:

// @flow
import type { xxxxxxxxxxx } from './defs';
export function myFunc(): xxxxxxxxxxx {
    return '42';
}

When adding // @flow to the defs.js file the error shows up. I propose that either:

• flow complains if the type file you require doesn't have the @flow pragma
• OR (even better) not require @flow to be there

See more about this feature here https://flowtype.org/docs/modules.html#type-imports--exports

[avikchaudhuri]

Sorry the docs don't make this point explicit!

[avikchaudhuri]

FWIW this behavior is not specific to import type but to all kinds of imports, but I agree it's particularly confusing for import type.

[houshuang]

I totally agree. I had a problem with a type imported from a different module, where I thought I had generated js.flow files, but it turned out I hadn't. If I explicitly import types, and it doesn't "work", I expect flow to shout loudly about it. In this case, I had imported the type, and type annotated code, and wondered why circle-ci passed the flow tests... Because I hadn't properly installed flow-copy-source.

[houshuang]

This issue just hit me again - I was trying to test a complex type configuration, and didn't understand why I couldn't get an error on an imported type, before finally realizing that I had inadvertently removed the //@flow annotation from the file exporting the type. When someone explicitly imports a type, you should definitively error if the type cannot be found.

[gcazaciuc]

Encountered this issue also multiple times and it's pretty hard to figure it out why the imported types just don't work. I believe a friendly error/warning would go a really long way in terms of smoothing the dev experience.

[rostero1]

If I import a type from a library that does not exist, I don't get any errors when I try to use that type, kind of defeating the point of flow.

[vicapow]

@rostero1 flow can catch these errors too but it’s a config you have to enable. See https://flow.org/en/docs/linting/rule-reference/#toc-untyped-type-import

[mrkev]

What's missing here? This is available via a lint: would better documentation on the default behaviour make this better? It's tagged as opt-in vs. opt-out: does this mean there's discussion wether to keep this this opt-in by default? (I personally think it should be opt-out).

[vicapow]

I do think we should consider making this an opt-out but might be better to move that to a separate ticket if there's interest