Magic functions you probably didn't know about

Magic BIFs and NIFs in OTP and Elixir

For the last few weeks I was working on purity and effects analyzer of Elixir and Erlang projects, and I came across a bunch of interesting functions which were built-in but undocumented or hidden.

Please, do not use them in production systems because they’re a subject to change
But don’t hesitate to play with them to check how the erlang runtime works, or make some dark magic there

  • :erts_internal.cmp_term compares two terms in oldschool -1, 0, 1 fashion
  • :erts_internal.term_type, function which returns a top-level type of the structure. Interesting thing is that for [] it returns nil and for nil it returns :atom. This function is really useful for debugging of binaries
  • :erts_internal.map_to_tuple_keys to show keys tuple for flatmaps (maps with <= 32 keys)
  • :erts_internal.map_hashmap_children to show the Tree representation of the hashmap (map with > 32 keys)
  • :erts_internal.ets_super_user to make the current process become a ETS process with maximum privileges. Process will have read/write access to any ets table (even protected and private ones)
  • :binary.referenced_byte_size (documented) to check a size of a referenced binary
  • :prim_eval.receve which works like a normal receive, but allows to decide whether to pick or not to pick the message from the queue to the closure rather then a pattern. It allows to peek in the head of the queue, without taking the value out of queue
  try do: :prim_eval.receive(fn x -> throw(x) end, 0), catch: (x -> x)
  • :erts_debug.same to check if the values are represented by the exactly same structure in memory (this means that pointers for heap values are equal)
  • :erts_debug.size and :erts_debug.flat_size to check the size with term reusage and flat size like if every term was referenced only once.

If you know any other magic functions, please post them in this thread :hugs:


I have a practical one:

:erlang.display_string '\a'



This one is documented but :erlang.phash2/2 can be quite useful to hash any arbitrary term as an integer within a range.

iex> :erlang.phash2({:works, %{with: ["just", "anything"]}}, 1000)
# default range of 2^27
iex> :erlang.phash2({:works, %{with: ["just", "anything"]}})