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:

```tomo
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` to get a non-null value by either providing an alternative non-null value
or by providing an early out statement like `return`/`skip`/`stop` or a function
with an `Abort` type like `fail()` or `exit()`:

```tomo
maybe_x := 5?
>> maybe_x or -1
= 5 : Int
>> maybe_x or fail("No value!")
= 5 : Int

maybe_x = !Int
>> maybe_x or -1
= -1 : Int
>> maybe_x or fail("No value!")
# Failure!

func do_stuff(matches:[Text]):
    pass

for line in lines:
    matches := line:matches($/{..},{..}/) or skip
    # The `or skip` above means that if we're here, `matches` is non-null:
    do_stuff(matches)
```