Flow equivalent to Java Wildcard? - flowtype

Question

Is there an equivalent to Java's Wildcards in Flow?
Here's my example code I've been working on as a test:
type InterfaceType = {
var1 : number,
};
type ActualType = InterfaceType & {
var2 : string,
};
type InterfaceGenericType<T : InterfaceType> = {
var3 : T,
}
type ActualGenericType = InterfaceGenericType<ActualType> & {
}
class State<T : InterfaceGenericType<InterfaceType>> {
prop : T;
constructor(arg : T) : State<T> {
this.prop = arg;
return this;
}
}
let actual : ActualType = {
var1: 1,
var2: "two",
};
let actualGeneric : ActualGenericType = {
var3 : actual,
}
let s2 = new State(actualGeneric);
This is the flow error I'm getting:
40: let s2 = new State(actualGeneric);
^ Cannot call `State` with `actualGeneric` bound to `arg` because property `var2` is missing in `InterfaceType` [1] but exists in object type [2] in property `var3`.
References:
20: class State<T : InterfaceGenericType<InterfaceType>> {
^ [1]
7: type ActualType = InterfaceType & {
^ [2]
I know I can get around the issue by doing:
class State<I : InterfaceType, T : InterfaceGenericType<I>> {
but I'm trying to not have to declare both types.

Answer 1

We can trim down your code a bit to remove the class:
type InterfaceType = { var1: number };
type ActualType = InterfaceType & { var2: string, };
type InterfaceGenericType<T : InterfaceType> = {
var3: T,
};
let actual: ActualType = {
var1: 1,
var2: "two",
};
let actualGeneric: InterfaceGenericType<ActualType> = {
var3: actual,
};
let v: InterfaceGenericType<InterfaceType> = actualGeneric;
I can't speak to Java since I don't know it, but I can tell you how to fix this. If we look at the error for this code:
17: let v: InterfaceGenericType<InterfaceType> = actualGeneric;
^ Cannot assign `actualGeneric` to `v` because property `var2` is missing in `InterfaceType` [1] but exists in object type [2] in type argument `T` [3].
References:
17: let v: InterfaceGenericType<InterfaceType> = actualGeneric;
^ [1]
2: type ActualType = InterfaceType & { var2: string, };
^ [2]
4: type InterfaceGenericType<T : InterfaceType> = {
^ [3]
The core issue is that v's type InterfaceGenericType<InterfaceType> would for example allow you to do
v.var3 = { var1: 42 };
because that is a valid InterfaceType object. That isn't a valid ActualType object, but by assigning actualGeneric to v, you've essentially erased that type information, which means that if your code were allowed as-is, the assignment would corrupt your actualGeneric object's type.
The fix for this is to tell Flow that the var3 property is read-only, by changing
var3: T,
to be
+var3: T,