Pattern match a list of any size in Elixir

Question

I'm new to Elixir. I want to define a function that accepts only lists, but that can be of any size.

I could define a function that accepts an empty list like this:

def doit(my_list = []) do
  IO.puts my_list
end

or just one item, like this:

def doit([first]) do
  IO.puts my_list
end

but how do I allow any size list? I know I can accept anything like this:

def doit(my_list) do
  IO.puts my_list
end

but wouldn't it be more correct to enforce that it is a list using pattern matching?

Answer 1

You can do it as below with pure pattern match and without the guards:

def any_list(list = [head | tail = []]) do IO.puts("Single Element") end

def any_list(list = [head | tail ]) do IO.puts("Any list") end

def any_list([]) do IO.puts("Empty list") end

Answer 2

As an alternative to pattern matching, you can use the is_list/1 guard function, which does a type check on the function argument:

def doit(my_list) when is_list(my_list) do
  # code goes here
end

Or even use it with the size check, if necessary:

def doit(my_list) when is_list(my_list) and length(my_list) > 0 do
  # code goes here
end

Answer 3

As far as I know there's no single pattern to match any list. [] will match an empty list and [_ | _] will match any non-empty list, but there's no way to combine them using a pattern.

You can do this with function guards though, using is_list/1:

def doit(my_list) when is_list(my_list) do
  # my_list is guaranteed to be a list
end