Treating `undefined` parameters as optional

[unional]

tsc: 2.2.0-dev.20161116

Discover this behavior from redux-utilities/flux-standard-action@78a9065

This is the original code:

export interface FSA<Payload, Meta> {
 ...
 payload?: Payload;
 ...
}

However, this does not work well with type guard:

function isSomeAction(action: any): action is FSA<{ message: string }, void> {
 return true;
}

let action = {...};
if (isSomeAction(action)) {
 // `action.payload` may be undefined.
 console.log(action.payload.message);
}

Since generic type can be anything, including undefined, I'm considering to remove the optional designation:

export interface FSA<Payload, Meta> {
 ...
 payload: Payload;
 ...
}

// now type guard works
let action = {...};
if (isSomeAction(action)) {
 console.log(action.payload.message);
}

However, now the creation code fail:

function createSomeAction(): FSA<undefined, undefined> {
 // error: `payload` and `meta` are required
 return { type: 'SOME_ACTION' };
}

The workaround is to depends on infer type:

function createSomeAction() {
 return { type: 'SOME_ACTION' };
}

or specify it:

function createSomeAction(): FSA<undefined, undefined> {
 return { 
 type: 'SOME_ACTION',
 payload: undefined,
 meta: undefined
 };
}

Is there a better solution? 🌷

[unional]

I understand that this is mixing two different contexts: Type of the value and type of the reference. But it would be nice to have a better solution to describe this situation?

[unional]

As a counter argument, it does make sense that the property should be not optional redux-utilities/flux-standard-action#45 (comment)

[unional]

A simpler example:

function createCounter<T>(x: number) {
 return (t: T) => {
 return x + 1
 }
}

const count = createCounter<undefined>(1)
count() // error, have to do `count(undefined)`

[unional]

Ryan Cavanaugh (@RyanCavanaugh) Mohamed Hegazy (@mhegazy) do you have any feedback on this? 🌷

Can this be covered in default generic type when specifying the default type is undefined?

[mhegazy]

There is nothing specific to generics here. for example:

declare function f(a: string | undefined): void;

f(); // Not allowed
f(undefined); // OK

[unional]

With generic defaults (#13487) landed, more people will encounter this issue. Should this be fixed?

i.e.:

export interface FSA<Payload = never> {
 payload: Payload;
}

export class SomeFSA implements FSA {
 // error. `payload` is missing
}