As first described here #8430

@adisbladis and me run a test where we reimplemented lib.findFirstIndex in terms of the new builtin.

  findFirstIndex =
    pred: default: list:
    let
      len = (length list);
      n = builtins.trampoline (
        idx:
        let
          found = pred (builtins.elemAt list idx);
        in
        [
          (if idx >= len then false else !found)
          (if idx >= len then -1 else if !found then idx + 1 else idx)
        ]
      ) 0;
    in
    if n == -1 then default else n;

Motivation

The idea was that findFirstIndex is exhaustive, running more iterations than necessary.
Using a trampoline it would be turned into O(n) worst case, from O(n) generally.

Observation

Running nix-instantiate pkgs.jq saved ~100 thunks

Takeway this might NOT be worth it - for the sake of exhaustive iterations in nixpkgs.
Maybe somebody has better use-case for it? I.e. the mentioned tail-recursing, or on bigger lists?

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