Type-Level Caching for Recursive Functions in TypeScript

This challenge focuses on implementing a type-level caching mechanism in TypeScript. The goal is to create a type that can memoize the results of a recursive type computation, preventing redundant calculations and improving type checking performance for complex recursive types. This is particularly useful for operations like deep type manipulation or complex generic function signatures.

Problem Description

You need to design and implement a TypeScript type called TypeCache that acts as a memoization decorator for other types. TypeCache should accept a type T as a generic argument. When TypeCache<T> is instantiated, it should store the result of resolving T. Subsequent instantiations with the same T should return the already computed (cached) result without re-evaluating T.

Key Requirements:

• The TypeCache type should be a generic type that takes one type argument, representing the type to be cached.
• It should effectively store and retrieve previously computed types.
• The mechanism should work for arbitrary, potentially deeply nested or recursive types.
• The cache should be managed entirely at the type level, without requiring any runtime JavaScript execution.

Expected Behavior:

When TypeCache<SomeType> is used, the type SomeType should be resolved. If SomeType has been resolved before via TypeCache<SomeType>, the previously resolved type should be returned immediately.

Edge Cases:

• Handling of primitive types.
• Handling of complex generic types.
• Handling of recursive types.

Examples

Example 1: Simple Caching

// Imagine a type that doubles a number at the type level
type Double<N extends number> = N extends number ? `${N}${N}` : never;

// Type-level cache for Double
type CachedDouble<N extends number> = TypeCache<Double<N>>;

type Result1 = CachedDouble<5>; // Expected: '55'
type Result2 = CachedDouble<5>; // Should be cached and resolve instantly

// If Double<5> was computationally expensive, Result2 would benefit.

Example 2: Caching Recursive Types

// A type to represent a linked list node
type ListNode<T, Next extends ListNode<any> | null = null> = {
    value: T;
    next: Next;
};

// A type that flattens a linked list into an array at the type level
type FlattenList<L extends ListNode<any> | null> =
    L extends ListNode<infer V, infer N>
        ? [V, ...FlattenList<N>]
        : [];

// Type-level cache for FlattenList
type CachedFlattenList<L extends ListNode<any> | null> = TypeCache<FlattenList<L>>;

type MyList = ListNode<number, ListNode<string, ListNode<boolean>>>;

type Flattened = CachedFlattenList<MyList>;
// Expected: [number, string, boolean]

type AnotherFlattened = CachedFlattenList<MyList>; // Should be cached

Example 3: Demonstrating Cache Hit

Consider a hypothetical ExpensiveType<T> that takes a significant amount of time for the TypeScript compiler to resolve.

// Assume this is a computationally intensive type
// For demonstration, we'll use a simpler type that just wraps another
type ExpensiveType<T> = { data: T };

// Our type-level cache
// type TypeCache<T> = ... your implementation ...

type CachedExpensive1 = TypeCache<ExpensiveType<string>>;
type CachedExpensive2 = TypeCache<ExpensiveType<string>>; // This should be a cache hit

// If the TypeScript compiler tracks which types have been resolved via TypeCache,
// CachedExpensive2 will resolve much faster than if it had to recompute ExpensiveType<string>.

Constraints

• The solution must be a TypeScript type.
• No runtime JavaScript code is allowed. The caching must occur entirely within the TypeScript type system.
• The TypeCache should ideally be generic enough to cache any valid TypeScript type.
• The implementation should aim for efficiency within the type system's capabilities.

Notes

Think about how TypeScript resolves types. It traverses a dependency graph. Your TypeCache needs to intercept this traversal and provide a shortcut when a type has already been computed and stored. Consider using intersection types, conditional types, and potentially mapped types to achieve this. The key challenge is to create a mechanism that effectively "remembers" the resolved form of a type. You might need a way to associate an input type with its resolved output type.