Is it possible to refer to a property of a type?

[philippotto]

I would like to write a generic setter function which receives the property name as a string and the property value as type T which should conform to the type determined by the property name.

Something like this would be nice:

/* @flow */

type t = {
  a: string,
  b: number,
}

const obj: t = {
  a: "hi",
  b: 1,
};

// Syntax error since t[k] doesn't work
function change<k: $Keys<t>>(key: k, value: t[k]) {
  obj[key] = value;
}

change("a", "...");
change("b", 1324);
change("b", "..."); // should fail

Is there a way to achieve this?

[agentcooper]

I've never actually used it, but probably $PropertyType is the thing you want: https://flow.org/en/docs/types/utilities/#toc-propertytype.

[philippotto]

Thanks for the pointer! I had looked through the utilities, but apparently I overlooked this.

However, I guess, I have to wait until $PropertyType allows the second parameter to be a non-string. Since this is sort of tracked in #2310, I'll close this issue.