Question about default arguments

In the book of Sasa Juriç in 2.3.3. there is this example

defmodule MyModule do
     def fun(a, b \\ 1, c, d \\ 2) do
          a+b+c+d
     end
end

Then it reads " So the previous code generates three functions: MyModule.fun/2,
MyModule.fun/3, and MyModule.fun/4"

Could somebody explain which functions would be generated in this case exactly?

TIA<
Arie

1 Like
def fun(a, c) do
  fun(a, 1, c)
end
def fun(a, b, c) do
  fun(a, b, c, 2)
end
def fun(a, b, c, d) do
  a + b + c + d
end
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Thx, That was not quite intuïtive for me!
In many languages optional parameters should be in last position, hence the confusion.

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So to be clear, it is not particularly idiomatic to mix required and optional parameters as the example you gave has. Sometimes you get an optional parameter at the beginning IE

def some_message(process_name \\ __MODULE__, arg) do
  GenServer.call(process_name, arg)
end

but the vast majority of the time it’s a best practice to leave defaults as the last argument(s). However as you observed the compiler is happy to handle any case.

2 Likes