Diff By Key

:::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 dict containing only the entries of the first KeyedTraversable whose keys do not appear in any of the other ones

namespace HH\Lib\Dict;

function diff_by_key<Tk1 as arraykey, Tk2 as arraykey, Tv>(
  KeyedTraversable<Tk1, Tv> $first,
  KeyedTraversable<Tk2, mixed> $second,
  KeyedContainer<Tk2, mixed> ...$rest,
): dict<Tk1, Tv>;

Time complexity: O(n + m), where n is size of $first and m is the combined size of $second plus all the ...$rest Space complexity: O(n + m), where n is size of $first and m is the combined size of $second plus all the ...$rest -- note that this is bigger than O(n)

Parameters

• KeyedTraversable<Tk1,Tv> $first
• KeyedTraversable<Tk2,mixed> $second
• KeyedContainer<Tk2,mixed> ...$rest

Returns

• dict<Tk1, Tv>