Default arguments best practices?

You can also use functions from the Keyword module

def foo(opts \\ []) do
    arg1 = Keyword.get(opts, :arg1, "default_value") 
    ...
end

I’ve learned the hard way that if you ever have more than one optional argument its a smell and you should refactor it to an opts list

amazing! @srcoulombe and @zachdaniel thanks for you advice

Absolutely never do this. Your colleagues or even your own future self will forget or be in a hurry just once and introduce a bug that would be difficult to track down.

Either only one optional argument and it must 99% of the time be the last one – unless the differences in type will make it completely obvious if something goes wrong – or use a keyword list as others have said. That approach also allows you to easily name stuff f.ex.

do_stuff(repo, changeset, username: "amir", password: "nope", dob: ~D[2000-01-01])

Or, using Keyword.validate!/2, would be better.

You have no idea how much I agree with this @zachdaniel, and it bites too many people in the butt too frequently. Maybe it should be part of the anti-patterns documentation!?

I’ve been working with Elixir for a long time now, how come I’ve missed this gem!? I surely must have overlooked it. Thanks for sharing!

Never say never . If param2 depends on param1, I don’t see why not.

Independent optional arguments should be in keyword list for sure.

I don’t see why you’d need multiple default arguments for that? You can detect presence with keyword lists and derive defaults etc.

Keyword.put_new_lazy(opts, :key, fn -> 
  other_value = opts[:other_key]
  some_function_of(other_value)
end)

I personally have found default arguments to be a good tool for trolling, as the default arguments are not restricted to the last arguments only as in many other languages, hence this is valid code too:

def(param1 \\ %{}, param2 \\ "", param3, param4 \\ []) do
# param3 is not optional
end

You know, it really bothers me knowing that KW lists require O(n) iteration over the list to extract the argument. I know full well that it will (almost) never matter in practice and I’m pretty sure it’s actually pushed down to native code, but I wince every time anyway…

Of course, I still use them frequently because named args can’t be beat for readability when you have a lot of them.