vitalydolgov
Pattern matching in `case` for maps
I have a question on pattern matching in case. This function
def f(map) do
case map do
%{a: i} -> i
%{} -> :empty
_ -> :other
end
end
works in this way:
> f(%{a: 67})
67
> f(%{b: 67})
:empty
> f(%{})
:empty
Can anybody explain me why it matches on the second clause, when non-empty map provided? I’m on Elixir 1.11.
Marked As Solved
Werner
It is also in the hexdocs the reference for it, in the section on maps:
“Note that the empty map will match all maps,…”
Also Liked
RudManusachi
Hi @vitalydolgov 
And welcome to the community!
%{} matches any map.
To match against empty map you could use either map_size/1 function or compare with %{} explicitly in guard as:
def f(map) do
case map do
# any map that has key :a
%{a: i} -> i
# empty map
map when map == %{} -> :empty
# map with exactly 1 key but not :a
map when map_size(map) == 1 -> :one_key_not_a
# any other map
%{} -> :other
# not a map
_ -> :not_a_map
end
end
dsounded
Hey
But basically you just need to use a guard for the second case
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