validation method like isString doesn't work when used in other methods having mixed arguments

[phoenisx]

Issue Code Example on /try

const isString = (string: mixed) => typeof string === 'string' || string instanceof String;

type Custom = {
  val: string,
};

const getCustom = (string: string): Custom => { return { val: string } }


function foo(x: string | Custom): Custom {
  const obj = isString(x) ? getCustom(x) : x;
  return obj;
}

foo() method above throws flow-type compilation errors Cannot call getCustom with x bound to string because Custom [1] is incompatible with string [2], though isString method validates x before any conversions

Above code works fine, if I replace isString(x), with it's definition:

// Works
function foo(x: string | Custom): Custom {
  const obj = typeof x === 'string' ? getCustom(x) : x;
  return obj;
}

[lll000111]

Flow Try hat works

function isString (thing: mixed): boolean %checks {
    return typeof thing === 'string';
}

Even using this special %checks keyword — of which you can learn if you search the issues (also closed ones), still undocumented but it seems you can rely on it to be a stable feature anyway — Flow only understands some constructs as type refinements but not others. I think arrow functions won't work. There is one trick you can use: In addition to the actual JS code you can just declare a function of the same name, in the example I use Flow comment syntax to do it inline in the JS file and not in a library file:

/*::
declare function isString(x: mixed): boolean %checks(typeof x === "string");
*/

Anther trick or hack that you can do is to write actual JS code, e.g. a function, in that Flow comment style. Flow will parse it as if it's part of the code, but the JS runtime will of course ignore it, since it's a comment. This feature can be used for all kinds of tricks (to get Flow to understand something), your imagination is the limit, without polluting your actual executable code..

Here's a Flow Try illustrating the declare solution.

Note that I have to use a function if I have that declare, if I use a variable I get a name conflict. Just a minor point and limitation. As you can see it now works without the %checks on the actual function, because Flow uses the definition it finds in the declare statement.

[phoenisx]

Thanks a lot @lll000111.

I did try to search open/closed issues (even googled it) before raising this issue, but I missed proper keywords for my search, thus couldn't find any solutions. I guess, it would be fine, if I raise a PR updating the docs related to function declares ?

Found that this feature is Documented as Predicate Functions, related to #4723

[lll000111]

The docs don't mention the Flow comment syntax or thedeclare hacks though. That's all stuff I only found by frequently reading issues and the responses.