What is the type of a type?

Question

What is the type of a type ?

This may all sound confusing, so a piece of code may say more than words.

// let's say there is some interface
interface A {
}

// and there is a class that implements it.
// there are a lot of implementations of A.
// but the AImpl is the current/best one.
class AImpl implements A {
}

// and then there is some kind of repository.
// which keeps track of these types, and knows which one is the best.
class Repository {

  // so, this repository returns the most current type.
  static getTypeForA(): Function {
    return AImpl;
  }
}

That's a bit strange, right ? It returns an instance of type Function, while actually it returns a class. It works fine, because a class is a function after all.

But there probably is something better, right? Which type should I use for a type?

What do you mean by `getTypeForA`? what are you expecting it to return? a `type`? or an implementation of interface `A`? — gilamran, Mar 19 '19 at 16:36
Can you elaborate what are you trying to achieve? what are you expecting `getTypeForA` to return (in run-time) ? — gilamran, Mar 19 '19 at 16:42
@Joe Clay, my bad. Tried to find one before, but couldn't. Thanks. - Ok for closing. — bvdb, Mar 19 '19 at 16:43
@gilamran see duplicate question. That is exactly what I am talking about. — bvdb, Mar 19 '19 at 16:44

gilamran · Answer 1 · 2019-03-19T16:38:02.060

0

You are probably expecting getTypeForA to return an implementation for interface A. so the return type should be the interface A.

interface A {
}

class Impl2 implements A {
}

class Impl1 implements A {
}

class Repository {

  static getImplForA(): A {
    if (condition) {
      return Impl1;
    }

    return Impl2;
  }
}

edited Mar 19 '19 at 16:38

answered Mar 19 '19 at 16:37

gilamran

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no I don't , I want it to return a Type, not an instance. – bvdb Mar 19 '19 at 16:37
types are for dev time only... – gilamran Mar 19 '19 at 16:39
yes, that is true. But the same goes for return types. All typing is indeed only present at dev time. (changes nothing) – bvdb Mar 19 '19 at 16:41
The function returns an instance of type `A` that will be determined in runtime. – gilamran Mar 19 '19 at 16:43

What is the type of a type?

1 Answers1