Issues With Deep Type Inference For OData Library

[molten pond]

I'm writing a typescript OData client query library and I'm stuck trying to get deep type inference to work. I have a query method that takes a query object that will be turned into a query string for the server request (mocked for this example). The expand property allows the caller to sub-query related entities and include them in the response.

Where im getting stuck is line 72 for the root of the issue, and line 51 for the error. Each expansion takes a function (for reasons described in comments) and the parameter is not able to be inferred (I think because typescript is having trouble with the function being an optional property).

Please let me know if you need more detail. I tried to document the code as best I could to describe my goals. I was worried a minimum reproduction would lose context and be more confusing to try and solve.

context:
OData is a http rest aligned standard for querying entities from an api. There is a tool that generates all the typescript models for me, but I wanted a library that helped me to:

• Build query strings in a type safe way
• Return type is the correct structure based on query

Being able to deeply infer the selections and expansions of the query is important because the api's return structure depends on what $select's (include entity properties) and $expand's (include related entities) you set.

[grim pecan]

_smidget#0

Playground Link

Preview:```ts
///////////////
//Test Models//
///////////////

type Person = {
//Properties
id: number
name: string
age: number

//Navigations
pets?: Animal[]

}

type Animal = {
//Properties
id: number
name: string
breed: Species
age: number
...```

You can choose specific lines to embed by selecting them before copying the link.

[molten pond]

!open

[molten pond]

!helper

I can put a minimum repro if the size is a problem

[slim cloud]

Arrow functions aren't contextually typed that is why you are getting that error.

If you really need pets to be optional what you could do is the following

type ODataExpansionEntry<T> =
  (q: ODataQueryFunction<T>) => ODataQuery<T>;

// musthave map (`-?` removes optionality here)
type ODataExpansionMap<TModel> = {
  [Prop in keyof ODataExpandModel<TModel>]-?: ODataExpansionEntry<ODataExpandModel<TModel>[Prop]>;
};

//make the map optional at the container level
type ODataExpansion<TModel> = Partial<ODataExpansionMap<TModel>>;