How to type check enums in switch statements

[gcanti]

This doesn't raise errors (seems a bug in Flow facebook/flow#1835):

type Action = { type: 'A' } | { type: 'B' };

export function foo(action: Action): number {
  switch (action.type) {
    case 'AA' : // <= typo
      return 1
    case 'B' :
      return 2
  }
  return 0
}

Can be alleviated by introducing an auxiliary enum and using a type cast in each case:

type ActionType = 'A' | 'B';
type Action = { type: 'A' } | { type: 'B' };

export function foo(action: Action): number {
  switch (action.type) {
    case ('AA': ActionType) :
      return 1
    case ('B': ActionType) :
      return 2
  }
  return 0
}

raises

src/index.js:8
  8:     case ('AA': ActionType) :
               ^^^^ string. This type is incompatible with
  8:     case ('AA': ActionType) :
                     ^^^^^^^^^^ string enum

Cons

• duplicated values in type ActionType

[gcanti]

Alas it doesn't work as expected, casting the enums seems to confuse Flow (v0.27.0) about object properties:

// @flow
type ActionType = 'A' | 'B';
type Action = { type: 'A', a: number } | { type: 'B', b: number };

export function foo(action: Action): number {
  switch (action.type) {
    case ('A': ActionType) :
      return action.a // <= throws property `a`. Property not found in object type
    case ('B': ActionType) :
      return action.b // <= throws property `b`. Property not found in object type
  }
  return 0
}