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unique_by

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​

Returns​

  • vec<Tv>