Why is data sharing in Elixir / Erlang not working for the same tuple in two maps

user_1483503 0
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Given the following code

defmodule Test do
  def run do
    p1 = {1, 2}
    m1 = %{a: p1}
    m2 = %{a: p1}
    IO.puts :erts_debug.same(m1.a, m2.a)
    m3 = %{b: p1}
    IO.puts :erts_debug.same(m1.a, m3.b)
  end
end

why does Test.run prints this

iex(1)> Test.run
true  <--- expected
false <--- not true ?!
:ok

Why are m1.a and m3.b not the same in-memory tuple?

elixir erlang
1 Answers

modern era update: seems like it was fixed in ≈v1.7.

This is true for Elixir only; in Erlang the tuple is shared:

1> Tuple = {1, 2},
1> Key1 = 1,
1> Key2 = 2,
1> Map1 = #{1 => Tuple, 2 => Tuple},
1> erts_debug:same(maps:get(Key1,Map1), maps:get(Key2,Map1)).
true

2> Key3 = 3,
2> Map2 = #{3 => Tuple},
2> erts_debug:same(maps:get(Key1,Map1), maps:get(Key3,Map2)).
true

For Elixir, this is probably because of internal transpiler to erlang duplicates maps or like. I’d say this could be a great bug report to Elixir core.

In your example :erts_debug.same(m1.a, m2.a) prints true only due to :erts_debug.same(m1, m2) #⇒ true, e.g. maps themselves share the same memory.

user_20352620
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