tomo/docs/optionals.md

Optional Values

A very common use case is values that may or may not be present. You could represent this case using enums like so:

enum MaybeInt(AnInt(x:Int), NoInt)

func maybe_takes_int(maybe_x:MaybeInt):
    when maybe_x is AnInt(x):
        say("Got an int: $x")
    else:
        say("Got nothing")

However, it's overly onerous to have to define a separate type for each situation where you might want to not have a value. Instead, Tomo has built-in support for optional types:

func maybe_takes_int(x:Int?):
    if x:
        say("Got an int: $x")
    else:
        say("Got nothing")

This establishes a common language for talking about optional values without having to use a more generalized form of enum which may have different naming conventions and which would generate a lot of unnecessary code.

In addition to using conditionals to check for null values, you can also use :or_else(fallback) or :or_fail():

maybe_x := 5?
>> maybe_x:or_else(-1)
= 5 : Int
>> maybe_x:or_fail()
= 5 : Int

maybe_x = !Int
>> maybe_x:or_else(-1)
= -1 : Int
>> maybe_x:or_fail()
# Failure!