Nostrum.Cache.GuildCache.select_by

You're seeing just the function select_by, go back to Nostrum.Cache.GuildCache module for more information.
Link to this function

select_by(clauses, selector)

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Specs

select_by(clauses(), selector()) :: {:ok, any()} | {:error, reason()}

Selects values using a selector from a Nostrum.Struct.Guild that matches the clauses.

Returns {:error, reason} if no result was found.

iex> Nostrum.Cache.GuildCache.select_by([id: 0], fn guild -> guild.id end)
{:ok, 0}

iex> Nostrum.Cache.GuildCache.select_by(%{id: 0}, fn guild -> guild.id end)
{:ok, 0}

iex> Nostrum.Cache.GuildCache.select_by([id: 10], fn guild -> guild.id end)
{:error, :id_not_found_on_guild_lookup}