Struggling with type inference, example codebase provided.

[elder spindle]

Hello,

I am trying to wrap my head around providing type inference to consumers while prototyping a type safe wrapper around some other libraries.

The issue I am having (along with accompanying codebase) is here: https://github.com/Tracktile/Axiom/blob/broken-provider/example/index.tsx

As you see, on line 31 User.schema is properly inferred.

However, on line 32 direct.User.schema is NOT properly inferred and receives a wider typing.

The difference, is that direct is the result of my createApi function which essentially binds my model objects to a query client. What this function does works, its the type inference of the result I am having issues with.

I want line 32 to be inferred to be the same type as 33.

Please let me know if you see what I've missed. I did have this working at some point, and have been starring at it for to long now.

[quartz spire]

i'm not sure

  M extends ModelFactoryMap<T, S>,
  T extends ModelFactory<S>,
  S extends TSchema
``` is a good idea

any reason youre not using this instead?

  M extends Record<string, ModelFactory<any>>

[elder spindle]

The only reason would be my own lack of understanding. With string as the record index type the object keys wouldn't be typed though, right?

ie. direct.<AnyString> would work, rather than only the keys for the models provided, which was my goal.

[quartz spire]

the keys would be typed, since the record is just the constraint for the generic parameter

[elder spindle]

Would M extends Record<keyof M, ModelFactory<any>> solve that?

Oh, I guess I have more to learn here.

I see now, because its in extends its not the same as a plain old type definition .

[quartz spire]

yea

the issue with M, T, S is that all your schemas must be the same type

because there's only one S

plus i'm guessing that it's hard for typescript to infer T and/or S, if it can even infer them at all

you can hover over createApi to check what the generic parameters are being inferred as

[elder spindle]

So in this type:

export type ModelMap<M extends Record<string, ModelFactory<any>>> = {
  [K in keyof M]: Model<S>;
};

What would Model<S> become? Since S is now longer a generic.

[quartz spire]

Model<M[K]> probably

well actually

i have no clue how ModelFactory and TSchema are related

[elder spindle]

ModelFactory is a function that takes a query client and returns a Model<T> where T must extend TSchema

[quartz spire]

so maybe ReturnType<M[K]>

[elder spindle]

YES.

Did did the trick:

export type ModelMap<M extends Record<string, ModelFactory<any>>> = {
  [K in keyof M]: ReturnType<M[K]>;
};

Thank you!!!!

ReturnType<M[K]> is Model<T>

[quartz spire]

type UnModelFactory<T> = T extends ModelFactory<infer U> ? U : never;

^- this is viable for more complicated generic types

it's also less reliable in certain cases (namely, cases where typescript forgets something is a ModelFactory - in this case ModelFactory is simple enough that it should never happen)

[quartz spire]

like, this may more explicit - but if a builtin type does the job that's just as good imo

[elder spindle]

Yeah, i just got it inferring through the react provider and hook I had in hooks.tsx as well, its working exactly as indended.