Problem with Literal Types extracted from an object using $Values<typeof OBJECT>

[ndbroadbent]

This is a:

• Bug Report
• Feature Request
• Question
• Other

Which concerns:

• flow-runtime
• babel-plugin-flow-runtime
• flow-runtime-validators
• flow-runtime-mobx
• flow-config-parser
• The documentation website

---

Background

I have been using an "enum" pattern with Flow that makes it a little easier to work with types.

First, I define some constants where they have a Literal Type for a specific string. Then I use these constants to build an object with some accessor keys. Finally, I use $Values to grab the types of each value and build a string enum for the type. Here's a real example from my application:

export const FIELD_TYPE_STRING: 'string' = 'string'
export const FIELD_TYPE_NUMBER: 'number' = 'number'
export const FIELD_TYPE_BOOLEAN: 'boolean' = 'boolean'
// ...

export const FIELD_TYPES = {
  STRING: FIELD_TYPE_STRING,
  NUMBER: FIELD_TYPE_NUMBER,
  BOOLEAN: FIELD_TYPE_BOOLEAN,
  // ...
}
export type FieldType = $Values<typeof FIELD_TYPES>

Then I can write code like this:

const type: FieldType = FIELD_TYPES.STRING

I can confirm that this pattern works fine with vanilla Flow, and Flow will raise an error if I try to assign "foo" to a variable with the FieldType type.

However, it looks like flow-runtime is struggling with these literal types, and it turns everything into string | string | string | ...

I am trying to see if I can use flow-runtime to automatically extract some of these enums and use them in other parts of my application (in a different programming language.)

What is the current behaviour?

Code (for https://gajus.github.io/flow-runtime/#/try)

import t, { reify } from 'flow-runtime'
import type { Type } from 'flow-runtime'

export const THING_TYPE_ONE: 'one' = 'one'
export const THING_TYPE_TWO: 'two' = 'two'
export const THING_TYPE_THREE: 'three' = 'three'

export const THING_TYPES = {
  ONE: THING_TYPE_ONE,
  TWO: THING_TYPE_TWO,
  THREE: THING_TYPE_THREE,
}
type ThingType = $Values<typeof THING_TYPES>

type Thing = {
  id: string | number;
  name: string;
  thing: ThingType;
};

const widget: Thing = {
  id: 123,
  name: 'Widget',
  thing: 'unknown'
};
console.log(widget);
console.log(ThingType.unwrap().toString())

Output:

{
  id: 123,
  name: "Widget",
  thing: "unknown"
}
string | string | string

---

What is the expected behaviour?

I am expecting the widget declaration to fail with an error like this:

Thing.thing must be one of: "one" | "two" | "three"

Expected: "one" | "two" | "three"

Actual Value: "unknown"

Actual Type: string

I am also expecting ThingType.unwrap().toString() to return "one" | "two" | "three".

Workaround

In the meantime, I could change my code and manually define the string enum:

export const FIELD_TYPE_STRING: 'string' = 'string'
export const FIELD_TYPE_NUMBER: 'number' = 'number'
export const FIELD_TYPE_BOOLEAN: 'boolean' = 'boolean'
// ...

export const FIELD_TYPES = {
  STRING: FIELD_TYPE_STRING,
  NUMBER: FIELD_TYPE_NUMBER,
  BOOLEAN: FIELD_TYPE_BOOLEAN,
  // ...
}
export type FieldType = 'string' | 'number' | 'boolean' | ...

But I would love to avoid the duplication and be able to use $Values to extract these Literal Types.

Which package versions are you using?

Latest versions on https://gajus.github.io/flow-runtime/#/try

Thank you!

[jedwards1211]

Doesn't ThingType.unwrap() cause Flow errors? I'm just curious, I had always used (reify: Type<ThingType>), .unwrap() would be nice but I assume it couldn't possibly work with Flow