For ets/mnesia guards, is there an :in equivelant?

Questions / Help
,
1

I am just starting to dabble with :mnesia.select/1 and for the most part it works fine, but how can I get the equivalent of Kernel.in/2 ?

For example this guard works and returns the correct results

[  {:==, :"$4", 10}  ]

But this aborts with a :badarg message, because erlang has no in (or maybe it does? I don’t know erlang)

[   {:in, :"$4", [10] }   ],

So how should I write such a guard clause?

In the erlang docs, I see that erlang DOES have a :lists.member function, but I cannot understand how to use that here. [ {:"lists.member", :"$4", [10]} ] does not appear to work, I’m not sure what syntax it is even expecting there.

According to the match spec docs, there is a set of built in functions, like ==, or map_get… But I see no way of asserting membership inside a list.

2

You have to compile them to a comparison with every element of the list using =:= and then or all comparisons together, which is precisely what the in macro does anyway. :slight_smile:

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3

Thanks Jose! That works.

For posterity, my implementation:

  def guard_in(value, li) do
    clauses = Enum.map(li, &({:"=:=", value, &1}))
    List.to_tuple([:or | clauses])
  end

  # ...

  :mnesia.transaction(fn ->
    :mnesia.select(table, [{
      {table, :_, :"$1"}, 
      [guard_in(:"$1", [10, 40])],
      [:"$$"]
    }])
  end)
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