Support predicate types

[sophiebits]

Currently, if you do

if (typeof x === "function") {
  // ...
}

then Flow knows that the code within the if statement is a function. This doesn't work if you instead call a helper function like Underscore's _.isFunction. Similarly, utilities like Object.assign and React.PropTypes seem to have hardcoded behavior at the AST level, so they don't work when aliased or given alternative names.

Will it be possible to teach Flow about these sorts of functions so that it's possible to use these third-party libraries without changing code too much?

[gabelevi]

Certainly we could hardcode this stuff in the typechecker, but I would love to come up with syntax to declare this kind of knowledge. We performed this kind of thought experiment with Hack without much success.

So for an isFunction(x) test, it would be great if we could declare that, if isFunction(x) returns true, then x is a function. Otherwise it is not a function. So maybe something like

declare function isFunction<T>(T x): true when T = function, false otherwise;

But that's kind of verbose. Maybe overloading can help?

declare function isFunction(x: function): true;
declare function isFunction(x: not function): false;

Ideally you'd also be able to declare when certain functions will throw, like in an assertFunction(x) function

declare function assertFunction<T extends function>(x: T): T;
declare function assertFunction(x: not function): explodes;

[sophiebits]

I just make problems; I don't fix them. :) I agree this probably isn't super trivial to fix.

[c-spencer]

The way this is tackled in things like core.typed is through a shorthand syntax for predicates on types. For example (ann function? (Pred Function)) defines a boolean return type with type refinement implications for the function? declaration. The full annotation would be along the lines of:

(ann function? (All [x] (IFn [x -> Boolean
                                   :filters {:then (is Function 0) :else (! Function 0)}])))

Where 0 refers to the first argument (x), and specifies refinements in each branch based on the return value. I don't think you'd ever get annotations like this to be particularly clean, though making them available certainly reduces the amount of special casing/privileged forms.

(I'm not really sure how you'd translate this kind of syntax to Flow, though having functions from types-to-types would seem to be valuable, so that declare function isFunction<T>(T x): Predicate(function); could return the properly annotated Boolean return type)

[spion]

TypeScript developers and users are currently discussing a similar feature in issue 1007

[jareware]

On a related note, since:

typeof null === 'object'

constraining types with code such as:

if (typeof o === 'object') {
    o.someProperty;
}

is not sufficient, whereas a simple _.isObject(o) would be.

I was actually really surprised Flow didn't catch the null issue I had in code such as the above, and only found it during testing.

[leebyron]

It looks like TypeScript ultimately decided on predicate function syntax, should flow try to use the same syntax?

function isB(p1: any): p1 is B;
(p1: any) => p1 is B;

Where the syntax in the return type could be Type | Predicate

Where Predicate := Identifier "is" Type

[samwgoldman]

@leebyron Cool! Can you provide a link to this discussion on TypeScript's side of things?

It's possible that Flow might have different capabilities and constraints around refinements that could affect the syntax here. For example, refinements can be anded and ored together, and flow can refine specific keys within object types.

Also, it seems like a function would always carry a (maybe void) return type, and optionally a refinement. Not either/or. For example, _.isFunction(x) still returns a boolean, but if the return value is true, x is an array. If it's false, x is not an array, and flow can use that information, too.

[leebyron]

microsoft/TypeScript#1007 is the issue discussing this, implemented in microsoft/TypeScript#3262.

https://github.com/Microsoft/TypeScript/pull/3262/files#diff-b09312f5c227031dfe4692c34364e2f0R16 this file is full of parser test cases.

[leebyron]

TypeScript's behavior seems to be that functions which return these type predicate refinements actually always return booleans.

I would propose that within the function, flow actually checks to ensure any possible "true" return agrees with the annotated type refinement.

Example, this should typecheck:

function isFunction(value: any): value is Function {
  if (typeof value === 'function') {
    return true;
  }
  return false;
}

var value: any = () => {};
if (isFunction(value)) {
  value();
}

This should fail:

function isFunction(value: any): value is Function {
  if (typeof value === 'function') {
    return true;
  }
  // Error: isFunction must return boolean
}

var value: any = () => {};
if (isFunction(value)) {
  value();
}

And this should fail:

function isFunction(value: any): value is Function {
  return true; // flow doesn't think "value" is Function here.
}

var value: any = () => {};
if (isFunction(value)) {
  value();
}

[samwgoldman]

Yeah, it seems like the x is y syntax is a boolean type with additional information (predicates that hold when the boolean is true).

This system is a good 80% solution, I think. We wouldn't be able to annotate the type of invariant, for example, because it returns void or blows up. I think that's OK.

I agree that flow should check that the return type is boolean, and it should also check that the truthiness of the returned value entails the correct refinement.

So, this would be bad:

function isFunction(x: any): x is Function {
  if (typeof value === 'function') {
    return false; // backwards
  }
  return true;
}

[leebyron]

Totally, flow should catch that backwards predicate.

This system is a good 80% solution, I think. We wouldn't be able to annotate the type of invariant, for example, because it returns void or blows up. I think that's OK.

Yeah, I think this is okay too. Invariant does something a little bit different anyhow, it may make sense to be a different scope of work.

...
invariant(isFunction(value));
value();

flow should be able to type check this correctly as well.