Flow: Generic type constraint ignored in higher-order function

[ghost]

Found a question on StackOverflow: https://stackoverflow.com/questions/49993018/flow-annotations-for-recursive-function/50002456

⭐️ How we'd like to write it using flow's generics

const Box = <A>(x:A): Box<A> => {
  return {
    map: <B>(f: A => B): Box<B> => Box(f(x)),
    fold: <B>(f: A => B): B => f(x),
  }
}

🤔 Now we create a demo number => string function with a sample Box<string>

const numberToString = (x: number) : string =>
  String (x)

const b : Box<string> =
  Box("1")

⚠️ This should fail, but it doesn't

b.map(numberToString)

🔍 Query is:issue is:open generic has 150+ open issues. If this is a duplicate or isn't considered a bug, feel free to close this issue.

🐛 View the bug here on flow.org/try

[apsavin]

Could you please provide a link to flow.org/try?

[ghost]

@apsavin i updated the post

[mrkev]

Hmm, this is unfortunate.
I'd suggest using classes as a workaround for now https://flow.org/try/#0MYGwhgzhAEBCD2APAPAQQHzQN4ChrUQC5pU9ph4A7CAFwCcBXYG+OgCiJIEptoaALAJYQAdImgBeAtAC+ZALZgADsljo2AM2KpJmWF2IIUa3nQCmNBnUrRKZgO5wkmtgOFiuPOfg3wQAE1V1LRJdOAM4UwsrGw1XIVFELxw5HApqGlsGeQAjMzoAFXgAZXpBSgBzSWgOYkpsvLoeYlo6cqqJdDJStsqapJw0qlpyaEMkZFb2zCk7RyM2ACIARkWuNJFFJTZ63Pyinvb1oA

[thehogfather]

@naomik @mrkev here is an alternative without classes. It is more verbose than the class version see flow try link here but I think the outcome is the same.

Main issue I spotted with the original post was the overloaded use of Box as a value and as an undeclared type (which probably means it becomes any hence no errors?).

type Box<A> = {
  map: <B>(f: A => B) => Box<B>,
  fold: <B>(f: A => B)=> B
}

const box = <A>(x:A): Box<A> => {
  return {
    map: <B>(f: A => B): Box<B> => box(f(x)),
    fold: <B>(f: A => B): B => f(x),
  }
}

const numberToString = (x: number) : string =>
  String (x)

const stringToNumber = (x: string): number => +x;

const b : Box<string> = box("1")

// should fail
b.map(numberToString)
//shoult not fail
const k = b.map(stringToNumber)

[mrkev]

I think I like that one better since it keeps more of the original semantics 👍

[Slowyn]

@thehogfather That what I was looking for in the original question. Your solution looks really cool 👍

[ghost]

@thehogfather thanks for the explanation, I didn't realize that type name can conflict with variable names in flow. It is indeed verbose but does exactly what is needed.