Flow: Does Flow require explicit annotations at the boundaries of modules ?

[leonardo-silva]

Hi. As far as I understood, if you make reference to a function that is implemented in another file (module) you need to explicitly annotate the types for the function before calling Flow type checker. Like in this example:

/**
 * Size.js
 * @flow
 */
function size(input: string): number {
  return input.length;
}

module.exports = size;

/**
 * UseSize.js
 * @flow
 */
var size = require('./Size');
var result = size('Hello Flow');

In this case, two files, we have to "manually" inform to Flow that 'function size' returns a 'number' and receives a 'string' as argument. Otherwise, it won't be able to infer the types and it will give a 'Missing annotation error' instead. Is this information correct? Do we really need to annotate the types manually in these cases?
Thank you in advance!

[drifkin]

Yep, annotations are required at module boundaries. See https://flowtype.org/docs/type-annotations.html#module-boundaries

Also see: https://flowtype.org/docs/modules.html#missingrequired-annotations

[Peeja]

This appears to have changed, as those links are now broken, but but there are still annotations which are required. Is there a new reference describing where annotations are required? I can't find it anywhere in the docs.