Introduce xs.merge type declaration

[xtianjohns]

Support type declarations when calling xs.merge(). This change allows consumers to leverage TypeScript to verify type safety when merging streams. It also brings merge in line with other static methods which already support this feature.

Description

Modify static merge method to implement a new interface: MergeSignature. This interface is constructed similarly to the CombineSignature interface. Export the new interface.

There are two areas I think merit discussion.

First: I'm not exactly sure what the intended behavior of this method is when called without arguments. The interface says a stream is returned that emits items of any type. And I suppose that's true. However, when I call this without any arguments, I get a stream that doesn't emit anything ever (or complete). Obviously my fault as the caller, but perhaps we want to do something else when it's called without arguments.

Second: I don't know how to test this or if it deserves a test. I imported core into a test file and was able to get type support for the method.

Motivation and Context

This should resolve #80.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[staltz]

I think the catch-all case should be

<T>(...stream: Array<Stream<T>>): Stream<T>;

Because that's what we had previously in the Stream class, under static merge etc.

[staltz]

First: I'm not exactly sure what the intended behavior of this method is when called without arguments. The interface says a stream is returned that emits items of any type. And I suppose that's true. However, when I call this without any arguments, I get a stream that doesn't emit anything ever (or complete). Obviously my fault as the caller, but perhaps we want to do something else when it's called without arguments.

As far as this PR is concerned, I don't think you need to change anything. It would be a breaking change, and we don't know if we want to do that.

Second: I don't know how to test this or if it deserves a test. I imported core into a test file and was able to get type support for the method.

Check this test for an inspiration: https://github.com/staltz/xstream/blob/master/tests/factory/combine.ts#L26

The trick is to define the type of the variable, e.g. const combined: Stream<[string, string]> at the same time as assigning to it. TypeScript compilation will fail (hence the test will fail) if the right-hand side of the assignment doesn't match the expected type on the left-hand side.

Nitpick: the commit message should follow our convention. Just use npm run commit instead of git commit. But anyway I can easily rename the commit myself.

PS: good PR. :)

[xtianjohns]

No problem - I'll get those fixed up this afternoon. Thanks!

[xtianjohns]

Alright, the catch all signature should look better now. I'm not sure what happened there.

Commit message should be improved, and when I amended the commit I threw in a test.

[staltz]

Perfect! Thanks!