Sanjibukai
Is it possible to partially pattern-match Lists as we do with Maps?
Hello everybody,
As it’s possible to partially pattern match with maps like so:
iex> %{b: 2} = %{a: 1, b: 2, c: 3}
%{a: 1, b: 2, c: 3}
Is it possible to do the same with Lists, e.g. like that:
iex> [2] = [1, 2, 3]
# MatchError
I guess that the inherent nature of linked lists (which make it easy to pattern-match heads/tails) might complicate things. Also I got the fact that in this case it might not make sense to pattern-match for variable binding . But it might be useful to pattern-match in this case for conditions/tests.
One can simply use Enum.member? but I don’t see an easy way to avoid conditions like we do when using pattern-matching in general or in functions argument.
Thank you very much for any details…
Marked As Solved
lucaong
Or even easier, as you can use it in guards:
def some_func(x) when x in [1, 2, 3] do
:present
end
def some_func(_) do
:not_present
end
Also Liked
NobbZ
Yeah, this different implemantation introduces slightly “bugs” if not understood correctly:
def f(x) when x in [1,2,3], do: "123"
def f(x) when x in 4..6, do: "456"
def f(_), do: "nope"
IO.puts f(2) #=> 123
IO.puts f(2.0) #=> nope
IO.puts f(2.5) #=> nope
IO.puts f(5) #=> 456
IO.puts f(5.0) #=> 456
IO.puts f(5.5) #=> 456
benwilson512
Hi @Sanjibukai you can match on the head and tail of a list, but there is no way to do a match which asks “Is this value anywhere in the list”.
[head | tail] = [1,2,3]
head #=> 1
tail #=> [2,3]
lucaong
As @benwilson512 said, you cannot pattern match in the middle of a list. If you are looking for a convenient and readable way to check for inclusion, you can do this though:
2 in [1, 2, 3]
You can also use it in guards:
case 2 do
x when x in [1, 2, 3] -> :yes
_ -> :no
end
lucaong
It’s interesting to notice how the guard is implemented. It is documented here and basically translates this:
when x in [1, 2, 3]
Into this:
when x === 1 or x === 2 or x === 3
The guard also works for ranges, with a different implementation, as the documentation shows.
lucaong
Important thing to notice in order to understand the behavior of in with ranges, is that a range is an Enumerable of integers. Therefore, (0..10) includes 5 but not 5.5, nor 5.0 (see the is_integer(x) in the guard expansion).
That is consistent with how Enum.member? works on ranges:
Enum.member?(0..10, 5) #=> true
Enum.member?(0..10, 5.5) #=> false
Enum.member?(0..10, 5.0) #=> false
In other words, it could be considered surprising, but it is not a bug, it is intended behavior.
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