Diff By

:::info[Note] This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information. :::

Returns a new vec containing only the elements of the first Traversable that do not appear in the second one, where an element's identity is determined by the scalar function

namespace HH\Lib\Vec;

function diff_by<Tv, Ts as arraykey>(
  Traversable<Tv> $first,
  Traversable<Tv> $second,
  (function(Tv): Ts) $scalar_func,
): vec<Tv>;

For vecs that contain arraykey elements, see Vec\diff().

Time complexity: O((n + m) * s), where n is the size of $first, m is the size of $second, and s is the complexity of $scalar_func Space complexity: O(n + m), where n is the size of $first and m is the size of $second -- note that this is bigger than O(n)

Parameters

• Traversable<Tv>$first
• Traversable<Tv>$second
• (function(Tv): Ts) $scalar_func

Returns

• vec<Tv>