`list -- list` causes unexpected result

hykw · November 25, 2020, 5:40am

I can reproduce the result like following. I wondered why the third one doesn’t result with []

iex(1)> x = [
20190122090654, 20190122090702, 20190122090705, 20190122090717, 20190124020608,
20190204062925, 20190204062926, 20190204063000, 20190204063001, 20190206064600,
20190314000000, 20190315000000, 20190705045039, 20190930010000, 20190930010001,
20190930010002, 20190930010003, 20190930010004, 20190930010005, 20201006015136,
20201009011409
]
iex(2)>  x == x
true
iex(3)> x -- x
[20190314000000, 20190930010005, 20201009011409]

Erlang: 21.2
Elixir 1.8.1

[note]
BTW, one day I’ve realized that executing mix ecto.migrations results below.
It was due to the result of list -- list issue, by the way.

  Status    Migration ID    Migration Name
--------------------------------------------------
<snip>
  up        20190930010000  create_meal_reports
  up        20190930010000  ** FILE NOT FOUND **
  up        20190930010001  create_meal_report_menu
  up        20190930010002  create_meal_report_claps
  up        20190930010003  create_meal_report_comments
  up        20190930010004  create_indexes_meal
  up        20190930010005  create_unique_indexes_meal
  up        20190930010005  ** FILE NOT FOUND **
  up        20201006015136  modify_qestion_type
  up        20201009011409  modify_meal_reports
  up        20201009011409  ** FILE NOT FOUND **

github.com

elixir-ecto/ecto_sql/blob/807246cd4b048417f350755153d7ad38b5ac294f/lib/ecto/migrator.ex#L461


  ups_with_file ++ ups_without_file ++ downs
end
defp versions_without_file(versions, migration_source) do
  versions_with_file =
    migration_source
    |> migrations_for()
    |> Enum.map(fn {version, _, _} -> version end)
  versions -- versions_with_file
end
defp lock_for_migrations(lock_or_migration_number, repo, opts, fun) do
  dynamic_repo = Keyword.get(opts, :dynamic_repo, repo)
  skip_table_creation = Keyword.get(opts, :skip_table_creation, false)
  previous_dynamic_repo = repo.put_dynamic_repo(dynamic_repo)
  try do
    config = repo.config()

Eiji · November 25, 2020, 5:49am

For me it looks different:

$ iex
Erlang/OTP 23 [erts-11.1.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Interactive Elixir (1.11.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> x = [                                      
...(1)> 20190122090654, 20190122090702, 20190122090705, 20190122090717, 20190124020608,
...(1)> 20190204062925, 20190204062926, 20190204063000, 20190204063001, 20190206064600,
...(1)> 20190314000000, 20190315000000, 20190705045039, 20190930010000, 20190930010001,
...(1)> 20190930010002, 20190930010003, 20190930010004, 20190930010005, 20201006015136,
...(1)> 20201009011409
...(1)> ]
[20190122090654, 20190122090702, 20190122090705, 20190122090717, 20190124020608,
 20190204062925, 20190204062926, 20190204063000, 20190204063001, 20190206064600,
 20190314000000, 20190315000000, 20190705045039, 20190930010000, 20190930010001,
 20190930010002, 20190930010003, 20190930010004, 20190930010005, 20201006015136,
 20201009011409]
iex(2)> x == x
true
iex(3)> x -- x
[]

Looks like you need to make an update. Unfortunately I did not found mention of fix for such bug in changelog and therefore I don’t know which versions are affected.

hykw · November 25, 2020, 6:26am

Hmm, now I’ve confirmed that it works as expected, say x -- x is [], in Elixir 1.11.2(Erlang 23.1.2)

Because I’ve also failed to find any description about the bug in changelog, I worry that it might occur in the future versions.

Eiji · November 25, 2020, 6:35am

Based on Elixir’s source code I guess that was an Erlang bug:

github.com

elixir-lang/elixir/blob/281d35e52062c06efcae10b05b3f0c905155e5b3/lib/elixir/lib/kernel.ex#L1403-L1406


      
          @spec list -- list :: list
          def left -- right do
            :erlang.--(left, right)
          end

Eiji · November 25, 2020, 6:38am

Possibly related: https://bugs.erlang.org/browse/ERL-986

hykw · November 25, 2020, 7:08am

Thank you. That’s it! As long as Erlang is older than the version(22.0.5 / 21.3.8.5), the issue still occurs even in the latest Elixir.

[NG]

Erlang 21.2.4
- Elixir 1.8.1
- Elixir 1.11.2

[OK]

Erlang 22.1
- Elixir 1.8.1
- Elixir 1.11.2
Erlang 23.1
- Elixir 1.11.2

benwilson512 · November 25, 2020, 2:09pm

And this folks is why property tests are great!