Is it possible to deduce a generic type from each element in a tuple?

I am currently facing an issue where I need the get function to limit the p parameter to the combination of all the types of key2 in the array. Additionally, key3 should be a keyof TFooKey.

type Foo<TFoo, TFooKey extends Record<string, any>> = {
    key1: TFooKey
    key2: TFoo
    key3: keyof TFooKey
}

// The goal is to have p as a union of all the types for key2 in the tuple
function get<TFoo, TFooKey extends Record<string, any>>(t: readonly Foo<TFoo, TFooKey>[], p: TFoo) {
    return t
}

// The issue arises when TFoo is only inferred from the first element
get(
[
    {
        key1: {
            key: ""
        },
        key2: "",
        key3: ""
    },
    {
        key1: {
            key: ""
        },
        key2: 0,
        key3: ""
    } 
] as const,
""
)

function get2<T extends readonly Foo<string | number, Record<string, any>>[]>(t: T, p: T[number]["key2"]) {
    return t
}

// While this solution works, it compromises on ensuring that key3 is a keyof TFooKey
get2(
[
    {
        key1: {
            key: ""
        },
        key2: "",
        key3: "key"
    },
    {
        key1: {
            key: ""
        },
        key2: 0,
        key3: "anyKey"
    } 
] as const, 
0
)

Above are the approaches I have tried and explained why they fall short. Is achieving this scenario even feasible?

Answer №1

When dealing with TypeScript's algorithm for inferring generic type arguments, it often does not synthesize unions from multiple competing candidate types. The scenario of matching an array against a generic type like `T[]`, where the array contains different data types such as strings, numbers, and booleans, can lead to inference issues due to the compiler selecting only one candidate (usually the first) and raising errors related to other candidates.

While this rejection behavior is intentional in some cases, there are situations where developers might prefer unions to be inferred automatically. An open issue on microsoft/TypeScript#44312 addresses this specific request for enhancing union inference capabilities in TypeScript, but currently, it is not a feature supported by the language.


To facilitate successful inference, adjusting the code to ensure there is only one candidate for the type argument and performing necessary type manipulations post-inference may be required. By constraining the generic type parameter T based on specific rules or conditions, developers can guide the compiler to deduce the correct union type.

One way to achieve this is by explicitly defining constraints within the function signature, outlining the acceptable types or properties that T must adhere to. This approach allows for a more precise control over the type inference process while accommodating complex scenarios involving unions and arrays.

Test out the modified implementation:

get([
    { key1: { key: "" }, key2: "", key3: "key" },
    { key1: { key: "" }, key2: 0, key3: "keyd" }
] as const, 0); // error!
// Type "keyd" is not assignable to type "key"

get([
    { key1: { key: "" }, key2: "", key3: "key" },
    { key1: { key: "" }, key2: 0, key3: "key" }
] as const, 0); // okay

The revised code ensures accurate type checking during function calls, signaling invalid combinations and approving valid ones.

Access the Playground link for hands-on experimentation

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