[T] tuple type information is lost as T[]

[nathancahill]

TypeScript Version:

nightly (1.9.0-dev.20160213)

Code

interface Geo {
    latlng: [number, number]  // tuple
}

function world(geo: Geo) {}

world({ latlng: [35, -112] });  // works fine

let coordinates = { latlng: [35, -112] };
world(coordinates);  // errors

let coordinatesTyped: Geo = { latlng: [35, -112] };
world(coordinatesTyped);  // works fine

Expected behavior:

Compile without errors.

Actual behavior:

Error, [T] tuple type information is lost as T[]:

Type 'number[]' is not assignable to type '[number, number]'.

[aluanhaddad]

The type information is not being lost.
In the line

let coordinates = { latlng: [35, -112] };

the inferred type of the object literal is

{ latlng: number[]; };

not Geo.
It is unrelated to the function call itself. When you define the object literal directly in the call as in

world({ latlng: [35, -112] });

it is target typed based on the function's parameter type to that more specific type.

While it is not entirely clear what the type of coordinates should be, note that

var t = [1, 2]; // t has type number[]
var [x, y] = [1, 2]; // x and y are of type number, there is no name for the tuple itself.

At any rate, neither the interface Geo nor the function world are taking part in type inference. They are red herrings.

[nathancahill]

My point is that [35, -112] could be number[] or [number, number]. The compiler is deciding that it's number[] and throwing an error when is used as [number, number].

[aluanhaddad]

Which is why I said it's not entirely clear what the type should be. Either one will be incompatible with certain signatures or assignments.

[aluanhaddad]

I suppose it could be

number[] & [number, number];

But that could get out of hand pretty quickly

[ahejlsberg]

This is by design. An array literal is considered a tuple only if it is contextually typed by a tuple type; for example, if it is assigned to a variable of a tuple type or passed as an argument for a parameter of a tuple type. In cases where the context provides no clear indication that it should be a tuple, we treat it as an array. It's not clear what else we would do, but we're certainly open to suggestions.

[aluanhaddad]

I think the current behavior matches the syntactic intuition that follows from the respective declaration forms for tuples and arrays.

[nathancahill]

Ok, thanks guys!