GrantJamesPowell
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)
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Nicd
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.Also Liked
wolf4earth
Or do
when map == %{}which has the same effect.lud
Map patterns match at least the given keys, so an empty map will match any map.
Aetherus
If you want to pattern match an empty map, you can use the guard
when map_size(a_map) == 0.Last Post!
aenglisc
The effect is the same but strict comparison should be more efficient I reckon.