Use function type shorthands when displaying signature help

From @unional on April 14, 2016 23:32

I would be great if when tape((<cursor>)) it would be able to drill in the tape.TestCase and show the signature of the callback. 🌷

This is probably on tsc, but let’s start here. 😄

Copied from original issue: Microsoft/vscode#5284

[DanielRosenwasser]

Sure thing @unional. All objects have a set of members. These aren’t the same things as properties. A property is a type of member, but members also include:

• call signatures
• construct signatures
• index signatures

The thing that allows a type to be callable is a call signature. You might already be familiar with call signatures if you’ve thought about overloads.

Here is an example of a type with a single call signature:

interface Foo {
    (hello: string, world: number): void;
}

If I have something of type Foo, I can call it with a string and number.

let f: Foo = /* ... */;

f("hello!", 100);

You can also have multiple call signatures.

interface Foo {
    (hello: string, world: number): void;
    (hello: string): void;
}

If Foo was declared like that, you could have also called f like so:

f("hello!");

---

We have two different shorter ways of writing things with call signatures. One place is method members.

If you have something like

interface Bar {
    m(): void;
}

Here, Bar is actually equivalent to

interface Bar {
    m: {
        (): void;
    }
}

Meaning that Bar has a property named m, which has a single call signature.

Another short-hand that we use is arrow function types. These are just objects with a single call signature.

In other words, if you’ve ever seen (x: number) => void, that’s actually just an easier way of writing the type { (x: number): void }.

Which is why I think that if you have an interface type with no members apart from a single call signature, we should consider showing the arrow function notation.

---

For more about call signatures, you can check out the language spec too: https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#3.9.2

[unional]

@DanielRosenwasser many thanks! Great examples. Especially that Bar and the arrow function one. Didn’t know function signature is expanded to a property with single call signature (in this case). It does make a lot of sense.