How to define a function with multiple clauses and optional parameters in elixir

Question

I'm trying to understand how to properly define a function with multiple clauses and optional parameters. When I write

  def sum([], total \\ 0), do: total
  def sum([h|t], total), do: h + sum(t, total)

(question Elixir default parameters for named functions with multiple clauses)

I get the warning:

warning: def sum/2 has multiple clauses and also declares default values. 
In such cases, the default values should be defined in a header. Instead of:

    def foo(:first_clause, b \\ :default) do ... end
    def foo(:second_clause, b) do ... end

one should write:

    def foo(a, b \\ :default)
    def foo(:first_clause, b) do ... end
    def foo(:second_clause, b) do ... end

I feel like the definition of sum follows this recommendation.

Could anyone explain why this is happening and what is the right way to define sum and eliminate the warning.

My version of elixir is 1.12.2.

Thanks in advance

score 7 · Accepted Answer · answered May 03 '22 at 13:13

7

Declare a head clause without do block with defaults

def sum(list, total \\ 0) # ⇐ THIS

def sum([], total), do: total
def sum([h | t], total), do: h + sum(t, total)

answered May 03 '22 at 13:13

Aleksei Matiushkin

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Just for reference, here is the [documentation for default arguments](https://hexdocs.pm/elixir/Kernel.html#def/2-default-arguments) which documents this use case. – sabiwara May 04 '22 at 00:23
1

Thanks for opening my eyes: it was "just" a matter of reading the warning and the doc properly! – mszmurlo May 04 '22 at 04:26
@mszmurlo always welcome, this occasional blindness happens to all of us. – Aleksei Matiushkin May 04 '22 at 06:40

How to define a function with multiple clauses and optional parameters in elixir

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