add "implements" keyword for functions

[exx8]

Search Terms

function, interface, function-type

Suggestion

I would like to add a "implements " option to functions.

Use Cases

Sometimes anonymous function or even named functions are in use in a further callback.
This is very prevalent in node.js.
Currently typescript does allow you to check variable which are functions, that they fit to a type.
It also allows you to check that a callback that you pass has a right signature.
But sometimes, whenever you write a module , you expose functions which will be used with different module which aren't available to you, (whether because the exposed function must be generic, or it hasn't been written or for a different reason).
The idiomatic way of ts to declare a function is something like this:

function my_pretty_function():void
{
}

Whereas had we wanted to enforce typing we needed to:
let my_pretty_function :my_beautiful_function_type=function():void
{
}
what I suggest is to add an implement keyword:

function implements my_beautiful_function_type my_pretty_function() :void
{
}

Examples

type num_type=()=>number;
function implements num_type my_pretty_function() :void
{
}

ts should print: signature mismatch, function my_pretty_function doesn't match num_type type.

Another interesting feature which might be implemented using this semantic addition is IDE auto complete.
Say you type:
function implements num_type square_root
IDEs will be able to auto-complete or suggest:

function implements num_type square_root():number
{
}

Checklist

My suggestion meets these guidelines:

• [ V] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [V ] This wouldn't change the runtime behavior of existing JavaScript code
• [V ] This could be implemented without emitting different JS based on the types of the expressions
• [ V] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• [V ] This feature would agree with the rest of TypeScript's Design Goals.

[MartinJohns]

Duplicate of #40378.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.