Unique 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 each element of the given Traversable exactly once, where uniqueness is determined by calling the given scalar function on the values

namespace HH\Lib\Vec;

function unique_by<Tv, Ts as arraykey>(
  Traversable<Tv> $traversable,
  (function(Tv): Ts) $scalar_func,
): vec<Tv>;

In case of duplicate scalar keys, later values will overwrite previous ones.

For arraykey elements, see Vec\unique().

Time complexity: O(n * s), where s is the complexity of $scalar_func Space complexity: O(n)

Parameters

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

Returns

• vec<Tv>