How do I fix this "Constructor cannot be called on object type" error in flow? - flowtype

Question

I'm having trouble figuring out the problem that flow is complaining about. I'm trying to allow the implementation of an API be changeable by storing the implementation class, then later instantiating it, however, flow complains when I call new this.implKlass saying that "Constructor cannot be called on object type". What is flow trying to tell me, and what am I conceptually missing about how flow works?
Example code below, and flow try code here
/* #flow */
type ApiT = {
fnA(): Promise<*>;
}
// An implementation of the API
class Impl {
async fnA(): Promise<*> { return 1; }
}
class DoThings {
implKlass: ApiT;
constructor(klass) {
this.implKlass = klass;
}
callA() {
const Klass = this.implKlass;
const inst = new Klass();
return inst.fnA();
}
}
new DoThings(Impl).callA();
Example output:
18: const inst = new Klass();
^ constructor call. Constructor cannot be called on
18: const inst = new Klass();
^ object type
13: constructor(klass: ApiT) {
^ property `fnA`. Property not found in
23: new DoThings(Impl).callA();
^ statics of Impl

Answer 1

With a small modification this works.
class DoThings {
implKlass: Class<ApiT>;
constructor(klass) {
this.implKlass = klass;
}
callA() {
const Klass = this.implKlass;
const inst = new Klass();
return inst.fnA();
}
}
The bug was you were writing ApiT instead of Class<ApiT>. ApiT would be an instance of a class, while Class<ApiT> is the class itself.
Try flow link

Answer 2

ApiT describes an object type, not a class type. An instance of the Impl class satisfies the ApiT type, but the class Impl itself does not. You cannot call Impl.fnA(), for example.
I'm not sure if there is any way to pass around constructors like this. However you can accomplish basically the same thing by using a factory function:
type ApiT = {
fnA(): Promise<*>;
}
type ApiTFactory = () => ApiT;
class Impl {
async fnA(): Promise<*> { return 1; }
}
class DoThings {
factory: ApiTFactory;
constructor(factory: ApiTFactory) {
this.factory = factory;
}
callA() {
const factory = this.factory;
const inst = factory();
return inst.fnA();
}
}
new DoThings(() => new Impl()).callA();
tryflow link