Add a LambdaTypeDeclare command

[JoshuaKGoldberg]

What command(s) would you like to add?

lambda type declare

Declares a type for a lambda that can be passed around similar to an interface.

Parameters

• export (optional): keyword to expose the lambda type declaration publicly
• typeName: PascalCase name of the type
• returnType: What type the lambda returns
• (parameterName, parameterType): Zero or more pairs of parameter names and types

Sample usage

lambda type declare : export FormatInt string value int

Why?

This is different from lambda type inline (#588), which is more akin to declaring a type primitive like null.
Languages need a prettier way than lambda type inline to declare lambda types, especially when many methods take in the same lambda type.

For example, in C#, FormatInt would be:

public delegate string FormatInt(int value);

...while in TypeScript, FormatInt would be:

type FormatInt = (value: number) => string;