Types passed to a higher-order function can get mixed up.

[dimvar]

Here, Flow warns that boolean and number are incompatible. Also, it warns that we may be passing a string to minus.

function app(f, x) {
  return f(x);
}
var call1 = app(function(x) { return x - 1; }, 123);
var call2 = app(function(x) { return x ? 'qwer' : 'adsf'; }, false);
call1 - 5;

[avikchaudhuri]

Same issue of Flow's inference not being polymorphic enough. Seems like we need to incorporate your CFA2 work into our analysis! :)

[dimvar]

I didn't realize that Flow analyzes function calls context insensitively, and that one has to annotate explicitly for polymorphism. Thanks for the tip, it explains a lot in my examples.

[ghost]

Thank you for reporting this issue and appreciate your patience. We've notified the core team for an update on this issue. We're looking for a response within the next 30 days or the issue may be closed.

[gcnew]

Is the following another instance of this issue?

function flip<A, B, R>(f: (a: A, b: B) => R): (b: B, a: A) => R {
    return (b, a) => f(a, b);
}

function fconst<A, B>(x: A, _: B): A {
    return x;
}

const g = flip(fconst);
const a1: string = g(1, "hello"); // error
const a2: number = g("hello", 1); // error

[lukeapage]

Adding typing to the hof does fix it.

https://flow.org/try/#0FAMwrgdgxgLglgewgAgIYAd0B4AqAaZAVQD4AKEALlJwEoBeYwggDwtosOQG9hlkAnAKYww-FCFLMaAbmABfYADdU-ZFFQAbDQEZkdNJnKRYiCJJrcBw0SmbIAtMm3TkcgtoBMAZhlKVazQ0PPQN0I2h4JHNLIRExZDsAfmQAcgBHAHdBfhTkClTUABMAZxAUlzdkEE1iwV91LV1HAFZpIA

Should this just be closed?

[SamChou19815]

Flow decides to require annotation for polymorphic functions a long time ago.