Appear only once

In this medium-level challenge, you'll implement FindEles<T> which filters a tuple to only include elements that appear exactly once in the array.

type cases = [
  Expect<Equal<FindEles<[1, 2, 2, 3, 3, 4, 5, 6, 6, 6]>, [1, 4, 5]>>,
  Expect<Equal<FindEles<[2, 2, 3, 3, 6, 6, 6]>, []>>,
  Expect<Equal<FindEles<[1, 2, 3]>, [1, 2, 3]>>,
  Expect<Equal<FindEles<[1, 2, number]>, [1, 2, number]>>,
  Expect<Equal<FindEles<[1, 2, number, number]>, [1, 2]>>,
]

Detailed Explanation

type Includes<T extends any[], V> = T extends [infer First, ...infer Rest]
  ? Equal<First, V> extends true
    ? true
    : Includes<Rest, V>
  : false
 
type FindEles<T extends any[], Seen extends any[] = []> = T extends [infer First, ...infer Rest]
  ? Includes<Rest, First> extends true
    ? FindEles<Rest, [...Seen, First]>
    : Includes<Seen, First> extends true
      ? FindEles<Rest, Seen>
      : [First, ...FindEles<Rest, Seen>]
  : []

How it works:

• Includes<T, V> is a helper type that checks whether a value V exists in a tuple T using strict Equal comparisons
• FindEles<T, Seen> processes the tuple element by element, maintaining a Seen accumulator of elements that have already been encountered as duplicates
• For each element First, it first checks if the same element appears later in Rest -- if so, the element is a duplicate, so it skips it and adds it to Seen
• If the element does not appear later but has already been Seen before, it is also skipped since it was previously identified as a duplicate
• Only if the element is neither found later nor previously seen is it included in the result with [First, ...FindEles<Rest, Seen>]
• The base case returns an empty tuple [] when all elements have been processed

This challenge helps you understand recursive tuple filtering with auxiliary tracking state, and how to apply these concepts in real-world scenarios.