My goal is to establish a compile-time constraint that ensures a variable of type T2 can be assigned to a variable of type T1.
If I have a value (t2) of type T2, I can simply do the following:
const t1: T1 = t2;
Is there an alternative method to achieve this without introducing additional runtime entities?
In my application, I do not directly define either of these types, so creating one as the superclass of the other is not an option.
Are you in need of a validator that will flag an error if T2 cannot be assigned to T1, all before runtime? Well, you can create something like this:
type ValidityWitness<U extends T, T> = U
This can be used as follows:
type T2Validation = ValidityWitness<T2, T1>
If T2 is indeed assignable to T1, no errors will pop up. However, if the assignment doesn't work, you'll receive an error specifying that T2 does not meet the constraint set by T1.
Let's demonstrate with some actual types:
interface Master {
key: string;
value: number;
}
interface GoodExtension {
key: 'x';
value: 5;
extra: boolean;
}
type AccurateValidation = ValidityWitness<GoodExtension, Master> // works fine
Note that GoodExtension isn't explicitly linked to Master, yet it is validated as a subtype nonetheless. Here's an example where validation fails:
interface FlawedExtension {
key: string;
extra: boolean;
}
type FaultyValidation = ValidityWitness<FlawedExtension, Master> // triggers an error
The compiler struggles to validate FlawedExtension as a subtype of Master, and the error message clearly points out the issue:
Type 'FlawedExtension' does not satisfy the constraint 'Master'. Property 'value' is missing in type 'FlawedExtension'.Hope this explanation proves beneficial! Best of luck!
It seems that achieving the desired outcome might not be possible in its current form. Apologies for the convoluted statement, but without a clear definition of the relative types, defining their relativity remains elusive.
Nevertheless, there are several alternative approaches available:
Utilize the any type for straightforward value assignment
const t1: T1 = <any>t2;
const t3: T2 = <any>t1;`
A similar solution involving a casting function:
function toT1(t: T2): T1 {
return <any>t;
}
const t1 = toT1(t2);
The benefit here is avoiding the need to explicitly define the type of t1.
Create a merged type combining two types into one
type T3 = T1 | T2;
const t1: T3 = t2;
This resolves the assignment dilemma but may introduce complications with entities already linked to T1 and T2. Use this approach selectively based on your requirements.
Develop a custom type guard:
interface T1 { a: any; }
interface T2 { b: any; }
function isT1(t: T1 | T2): t is T1 {
return true;
}
const t2: T2 = { b: 1 };
const t1: T1 = isT1(t2) ? t2 : null;
This always returns true, allowing you to assign your variable to t1 smoothly. Nonetheless, this method still has drawbacks.
I recommend opting for method number 1. It offers simplicity while adequately addressing your requirements. Remember, excessive complexity can be detrimental.
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