Elixir：为什么会出现“未定义函数”错误？

Question

The Elixir compiler gives me "undefined function changes/0" in the third line of the function:

  def ignore_empty(changeset) do
    cond do
      %{valid?: false, changes: changes} = changeset when changes == %{} -> 
        %{changeset | action: :ignore}
      true -> changeset  
    end    
  end 

Why is that? I mostly copied this code from Ecto.Changeset documentation of the cast_assoc function here

Answer 1

You probably meant to use case , eg

def ignore_empty(changeset) do
  case changeset do
    %{valid?: false, changes: changes} = changeset when changes == %{} ->
      %{changeset | action: :ignore}
    _ ->
      changeset
  end
end

The reason you are getting "undefined function" is because when the expression is getting expanded, that happens to be the first error the compiler encounters. It's valid to put an expression in a cond , so other than the reference to an undefined function changes the other thing which it would fail on is the use of when . In any case, that appears to be the issue here.