Modular•6mo ago

Optional function argument?

Hi everyone I'm wondering if it's possible to have an optional argument for functions in Mojo. So then if an argument were given, it would have a value (int for instance) otherwise, it would be None.

3 Replies

ModularBot•6mo ago

Congrats @Arvin_David, you just advanced to level 1!

sora•6mo ago

There are three ways to do this: - default argument using the Optional type

fn f(a: Optional[Int] = None):
  ...

fn f(a: Optional[Int] = None):
  ...

- default argument

fn f(a: Int = default_value):
  ...

fn f(a: Int = default_value):
  ...

- overload

fn f():
  f(default_value)
fn f(a: Int):
  ...

fn f():
  f(default_value)
fn f(a: Int):
  ...

We don't have Python's mutable default problem, so using a Optional type is unnecessary in many cases.

Arvin_David•6mo ago

Thanks a lot. But how can I also check if it's none or not?

from collections.optional import Optional

def main():
  test(3)

fn test(item0: Int, item1: Optional[Int] = None):
  print(item1.value())

from collections.optional import Optional

def main():
  test(3)

fn test(item0: Int, item1: Optional[Int] = None):
  print(item1.value())

For example in the code above, I wonder why the printed value is 0. As far as I know, we cannot print None at the moment, So is mojo converting None to 0, or None was initially treated like zero? How can I check if it's originally None or not? I found the solution. It's a little bit tricky but cool to know:

from collections.optional import Optional

def main():
  test(3) # does not print anything
  test(3, 0) # prints 0
  test(3, 5) # prints 5

fn test(item0: Int, item1: Optional[Int] = None):
  if item1:
     print(item1.value())

from collections.optional import Optional

def main():
  test(3) # does not print anything
  test(3, 0) # prints 0
  test(3, 5) # prints 5

fn test(item0: Int, item1: Optional[Int] = None):
  if item1:
     print(item1.value())

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Optional function argument?