# ETS - conditional match with variable in Elixir **URL:** \ **Category:** Questions\ **Tags:** ets, troubleshooting\ **Created:** [November 24, 2022, 3:45pm UTC](https://forum.elixirforum.com/t/ets-conditional-match-with-variable-in-elixir/52010 "2022-11-24T15:45:10Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![ndrean](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/ndrean/32/36680_2.png) [@ndrean](https://forum.elixirforum.com/u/ndrean)\ **Post date:** [November 24, 2022, 3:45pm UTC](https://forum.elixirforum.com/t/ets-conditional-match-with-variable-in-elixir/52010/1 "2022-11-24T15:45:10Z")
I want to query an ETS table based on a condition. As per [Erlang’s doc](https://www.erlang.org/doc/man/ets.html#fun2ms-1), I should be able to use variables in scope. Is it possible to formulate the query below with this “imported variable” in Elixir ? ```elixir def get(id) do fun = :ets.fun2ms(fn {x,y,z,:_} when x == id or y == id -> {x,y,z} end) :ets.select(:table, fun) end ``` This does not work with a variable, but works with a value. ```elixir (FunctionClauseError) no function clause matching in :ms_transform.normalise/1 ``` You can obtain the result as below but not satisfied ```elixir def get(id), do: [ :ets.match_object(:table, {:"$1", id, :"$2", :_}) | :ets.match_object(:table, {:"$1", :"$2", id, :_}) ] |> List.flatten() ``` ---
**Author:** ![LostKobrakai](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/lostkobrakai/32/3072_2.png) [@LostKobrakai](https://forum.elixirforum.com/u/LostKobrakai)\ **Post date:** [November 24, 2022, 3:59pm UTC](https://forum.elixirforum.com/t/ets-conditional-match-with-variable-in-elixir/52010/2 "2022-11-24T15:59:33Z")
I’m not sure why exactly, but `:ets.fun2ms` doesn’t work in elixir (or at least not in all contexts). There’s `:ex2ms` to solve this: ```elixir import Ex2ms fun do {x,y,z,_} when x == 1 or y == 1 -> {x,y,z} end # [ # {{:"$1", :"$2", :"$3", :_}, [{:orelse, {:==, :"$1", 1}, {:==, :"$2", 1}}], # [{{:"$1", :"$2", :"$3"}}]} # ] ``` Replace `1` with `id`. ---
**Author:** ![ndrean](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/ndrean/32/36680_2.png) [@ndrean](https://forum.elixirforum.com/u/ndrean)\ **Post date:** [November 24, 2022, 4:14pm UTC](https://forum.elixirforum.com/t/ets-conditional-match-with-variable-in-elixir/52010/3 "2022-11-24T16:14:27Z")
Yes this produces the “desired” 🙂 pattern. Thks ```elixir q = [{ {:"$1", :"$2", :"$3", :_}, [{:orelse, {:==, :"$2", {:const, id}}, {:==, :"$3", {:const, id}}}], [{{:"$1", :"$2", :"$3"}}] }] ``` with: ```elixir q = Ex2ms.fun do {ch, e, r, _} when e == ^id or r == ^id -> {ch, e, r} end ``` ---
**Author:** ![al2o3cr](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/al2o3cr/32/3457_2.png) [@al2o3cr](https://forum.elixirforum.com/u/al2o3cr)\ **Post date:** [November 25, 2022, 1:57am UTC](https://forum.elixirforum.com/t/ets-conditional-match-with-variable-in-elixir/52010/4 "2022-11-25T01:57:23Z")
> [@LostKobrakai](#): > > I’m not sure why exactly, but `:ets.fun2ms` doesn’t work in elixir Hmmm, the full stack trace is interesting: ```elixir iex(7)> :ets.fun2ms(fn {x,y,z,:_} when x == id or y == id -> {x,y,z} end) ** (FunctionClauseError) no function clause matching in :ms_transform.normalise/1 The following arguments were given to :ms_transform.normalise/1: # 1 {:EXIT, {:badarg, [ {:lists, :keysearch, [:_@0, 1, %{_@0: 4}], [error_info: %{module: :erl_stdlib_errors}]}, {:ms_transform, :fixup_environment, 2, [file: 'ms_transform.erl', line: 1104]}, {:lists, :map_1, 2, [file: 'lists.erl', line: 1320]}, {:lists, :map, 2, [file: 'lists.erl', line: 1315]}, {:lists, :map_1, 2, [file: 'lists.erl', line: 1320]}, {:lists, :map_1, 2, [file: 'lists.erl', line: 1320]}, {:lists, :map, 2, [file: 'lists.erl', line: 1315]}, {:ms_transform, :fixup_environment, 2, [file: 'ms_transform.erl', line: 1112]} ]}} (stdlib 4.1.1) ms_transform.erl:1124: :ms_transform.normalise/1 (stdlib 4.1.1) ms_transform.erl:222: :ms_transform.transform_from_shell/3 (stdlib 4.1.1) ets.erl:605: :ets.fun2ms/1 iex:7: (file) ``` There’s actually TWO crashes happening here: - `:ms_transform.normalize` is being called with an `{:EXIT, ...etc}` tuple because the error handling in `transform_from_shell` wasn’t expecting that shape: [https://github.com/erlang/otp/blob/d95d79d1e363879462d9775316896c02b59771ee/lib/stdlib/src/ms\_transform.erl#L217-L225](https://github.com/erlang/otp/blob/d95d79d1e363879462d9775316896c02b59771ee/lib/stdlib/src/ms_transform.erl#L217-L225) - that tuple is coming from `:ms_transform.fixup_environment`, which expects an environment `B` as a keyword list but gets a _map_ instead: [https://github.com/erlang/otp/blob/d95d79d1e363879462d9775316896c02b59771ee/lib/stdlib/src/ms\_transform.erl#L1103-L1109](https://github.com/erlang/otp/blob/d95d79d1e363879462d9775316896c02b59771ee/lib/stdlib/src/ms_transform.erl#L1103-L1109) The root cause is that `:erl_eval.fun_data/1` is returning a map for the bound variables 🤔 : ```elixir iex(9)> :erl_eval.fun_data(fn {x,y,z,:_} when x == id or y == id -> {x,y,z} end) {:fun_data, %{_@0: 4}, [ {:clause, 9, [ {:tuple, 9, [{:var, 9, :_x@1}, {:var, 9, :_y@1}, {:var, 9, :_z@1}, {:atom, 9, :_}]} ], [ [ {:op, 9, :orelse, {:op, 9, :==, {:var, 9, :_x@1}, {:var, 9, :_@0}}, {:op, 9, :==, {:var, 9, :_y@1}, {:var, 9, :_@0}}} ] ], [{:tuple, 9, [{:var, 9, :_x@1}, {:var, 9, :_y@1}, {:var, 9, :_z@1}]}]} ]} ``` ---
**Author:** ![krstfk](https://forum.elixirforum.com/letter_avatar_proxy/v4/letter/k/c67d28/32.png) [@krstfk](https://forum.elixirforum.com/u/krstfk)\ **Post date:** [November 25, 2022, 8:28am UTC](https://forum.elixirforum.com/t/ets-conditional-match-with-variable-in-elixir/52010/5 "2022-11-25T08:28:26Z")
`fun2ms` and friends are parse transformations, they can be used from the shell, but the transformation is applied at compile time using `ms_transform.hrl` which cannot really be done in elixir. See [the full documentation](https://www.erlang.org/doc/man/ms_transform.html) especially the warnings at the end of the documentation.