Pattern Match On Empty Maps

I’m really confused why the an empty map pattern matches with a random other map

%{} = %{foo: :bar}

but now when set to a variable

empty_map = %{}
^empty_map = %{foo: :bar}
** (MatchError) no match of right hand side value: %{foo: :bar}
    (stdlib) erl_eval.erl:453: :erl_eval.expr/5
    (iex) lib/iex/evaluator.ex:257: IEx.Evaluator.handle_eval/5
    (iex) lib/iex/evaluator.ex:237: IEx.Evaluator.do_eval/3
    (iex) lib/iex/evaluator.ex:215: IEx.Evaluator.eval/3
    (iex) lib/iex/evaluator.ex:103: IEx.Evaluator.loop/1
    (iex) lib/iex/evaluator.ex:27: IEx.Evaluator.init/4

Reproducible on Mac OSX with the following Elixir/Erlang

Erlang/OTP 22 [erts-10.7.2.1] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Interactive Elixir (1.9.4) - press Ctrl+C to exit (type h() ENTER for help)
1 Like

Reproducible on Pop!_OS too. I guess a pinned variable expects an exact match.

my_map = %{foo: :bar}
^my_map = %{foo: :bar, baz: :qux}
** (MatchError) no match of right hand side value: %{baz: :qux, foo: :bar}

This behaviour shows in Erlang, too

#{foo := bar} = #{foo => bar, baz => qux}.
returns #{baz => qux,foo => bar}

MyMap = #{foo => bar}.
MyMap = #{foo => bar, baz => qux}.
** exception error: no match of right hand side value #{baz => qux,foo => bar}
1 Like

This happens because the pin is used to match a value in a pattern. The value in this case is %{} and the RHS doesn’t match it. Patterns are not first class in Elixir or Erlang, so you cannot store them in variables and use for matching.

7 Likes

Map patterns match at least the given keys, so an empty map will match any map.

2 Likes

If you want to pattern match an empty map, you can use the guard when map_size(a_map) == 0.

1 Like

Or do when map == %{} which has the same effect.

2 Likes

Thanks for the help. Nicd points out the thing I was missing in that patterns aren’t first class.

Anyone who comes here in the future who wants to have a function with a dynamic match this is how I ended up doing it

def get_items(filters \\ %{}) do
  filters = MapSet.new(filters)

  for item <- get_all_items(),
    MapSet.subset?(filters, MapSet.new(item)),
    do: item
end

The effect is the same but strict comparison should be more efficient I reckon.