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This is available as Dict\group_by in the www repository.

Return a dict keyed by the result of calling the giving function, preserving duplicate values

namespace HH\Lib\Dict;

function group_by<Tk as arraykey, Tv>(
  Traversable<Tv> $values,
  (function(Tv): ?Tk) $key_func,
): dict<Tk, vec<Tv>>;
  • keys are the results of the given function called on the given values.
  • values are vecs of original values that all produced the same key.

If a value produces a null key, it's omitted from the result.

Time complexity: O(n * f), where f is the complexity of $key_func Space complexity: O(n)



  • dict<Tk, vec<Tv>>


Group numbers by their parity (separate even and odd numbers):

$numbers = vec[1, 1, 2, 3, 5, 8, 14];
$groups = Dict\group_by($numbers, $value ==> $value % 2);