Pattern match function against empty map
I'm playing around with pattern match and I found out, that it's not quite easy to pattern match parameters of a method against an empty map. I thought it would go something like this:
defmodule PatternMatch do
def modify(%{}) do
%{}
end
def modify(map) do
# expensive operation
%{ modified: "map" }
end
end
But it seems like the first function clause matches arbitrary maps:
iex> PatternMatch.modify(%{a: "map"})
==> %{}
Is there another way to check for empty maps?
Solution 1:
It works this way by design, but admittedly it can be a bit confusing at first glance. This feature allows you to destructure maps using pattern matching, without having to specify all keys. For example:
iex> %{b: value} = %{a: 1, b: 2, c: 3}
%{a: 1, b: 2, c: 3}
iex> value
2
Consequently, %{}
will match any map. If you want to match an empty map in a function, you have to use a guard clause:
defmodule PatternMatch do
def modify(map) when map == %{} do
%{}
end
def modify(map) do
# ...
end
end
Solution 2:
In addition to @PatrickOscity's answer (which I would use for an empty map), you can use a map_size/1 guard to match against maps with a number of keys:
defmodule PatternMatch do
def modify(map) when map_size(map) == 0 do
%{}
end
def modify(map) when map_size(map) == 1 do
#something else
end
def modify(map) do
# expensive operation
%{ modified: "map" }
end
end
Here is an output from iex using Kernel.match?/2
to show map_size/1
in action:
iex(6)> Kernel.match?(map when map_size(map) == 1, %{})
false
iex(7)> Kernel.match?(map when map_size(map) == 1, %{foo: "bar"})
true