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Unions of objects/dictionaries/maps and tuples generate type errors when numeric keys are present #13179
Description
TypeScript Version: 2.1.4
Code
type BoxOfBoolPairs = { [name: string]: BoxOfBoolPairs } | [boolean, boolean]
// Good.
const boxA : BoxOfBoolPairs = {}
// Good.
const boxB : BoxOfBoolPairs = { hey: [true, false], world: [false, true] }
// Good.
const boxC : BoxOfBoolPairs = { hey: [true, false], world: { earth: [true, true], notEarth: [false, false] } }
// Good.
const boxD : BoxOfBoolPairs = { "hey": [true, false], "world": [false, true] }
// Type error.
const boxE : BoxOfBoolPairs = { hey: [true, false], world: [false, true], "0": [false, true] }Expected behavior:
boxE is seen as an object containing tuples "hey", "world" and "0".
Actual behavior:
[ts]
Type '{ "0": boolean[]; hey: [true, false]; world: [false, true]; }' is not assignable to type 'BoxOfBoolPairs'.
Type '{ "0": boolean[]; hey: [true, false]; world: [false, true]; }' is not assignable to type '[boolean, boolean]'.
Types of property '0' are incompatible.
Type 'boolean[]' is not assignable to type 'boolean'.
Activity
sandersn commentedon Apr 24, 2017
The error message is not the best here, which is a problem we have with unions from time to time. The compiler thinks things that have a
"0"property are tuples, so it has comparedboxEto the[boolean, boolean]branch ofBoxOfBoolPairs, not the string indexer branch.That's because tuples are really just aliases for a family of types that look kind of like this:
So what happens is that the compiler sees the '0', tries to infer
Tfrom[false, true], tries to inferUfrom the non-existent '1' property and creates aTuple<boolean[], {}>. This fails to matchTuple<boolean, boolean>and creates an error.I'll go see why the compiler doesn't explore the string indexer branch and find a match.
sandersn commentedon Apr 25, 2017
Looks like contextual type applies to
heyto give the type[true, false]but0has the contextual typeboolean[], which is then not assignable to either side ofBoxOfBoolPairs.The type of '0' is taken from the contextual type
BoxOfBoolPairs, which finds a property'0': booleanin the[boolean, boolean]side of the union.The type of 'hey' is taken from the contextual type
BoxOfBookPairs, which does not find a matching propertyhey, so falls back on the string indexer type.I need to think about a fix; finding a specific property name
0should definitely outweigh the type of the string indexer. But it does the wrong thing in this case.sandersn commentedon Apr 25, 2017
After talking to Ryan Cavanaugh (@RyanCavanaugh), I don't think there is a way to create values of type
BoxOfBoolPairsin Typescript. Because of the way tuples and arrays are defined in Javascript as objects with number-string keys, it's not possible to distinguish the two sides of the union when given a value.The correct fix is to use the ADT pattern and add a discriminant field, something like
kind, to distinguish between leaf and non-leaf instances of the type.