🐥 Fixing default argument errors in multi-clause functions in Elixir

When working with Elixir, you might encounter an error when using default arguments in multiple function clauses. Let’s break down why this happens and how to fix it.

Consider the following Elixir code:

def change_post(%Post{id: nil} = post, attrs \\ %{}) do
  Post.changeset(post, attrs) |> Ecto.Changeset.put_assoc(:taggings, [])
end

def change_post(%Post{} = post, attrs \\ %{}) do
  Post.changeset(post, attrs)
end

At first glance, this looks fine. However, Elixir will raise an error because default arguments (\\ %{}) are only allowed in a single function head. Defining them in multiple clauses is invalid.

To fix this, move the default argument to a single function head and delegate calls internally:

def change_post(post, attrs \\ %{})

def change_post(%Post{id: nil} = post, attrs) do
  Post.changeset(post, attrs) |> Ecto.Changeset.put_assoc(:taggings, [])
end

def change_post(%Post{} = post, attrs) do
  Post.changeset(post, attrs)
end

Why does this work?

• The first line (def change_post(post, attrs \\ %{})) ensures the default argument is only set once.
• The subsequent clauses pattern match on post without redefining the default.
• Elixir correctly applies the default argument when attrs is not provided.

Whenever you need multiple function clauses but also want a default argument, define the default once and delegate matching to separate clauses. This keeps your code clean, idiomatic, and error-free.