56 lines
1.1 KiB
Tcl
56 lines
1.1 KiB
Tcl
# Optional Types
|
|
|
|
func main():
|
|
|
|
# Any type `T` can be made optional with the syntax `T?`,
|
|
# meaning it can hold a value or be `none`.
|
|
|
|
x := 42?
|
|
>> x
|
|
= 42
|
|
|
|
# You can assign a `none` value to `x` because it has type `Int?`
|
|
x = none
|
|
|
|
# To declare a `none` variable, specify its type:
|
|
y := none:Int
|
|
|
|
>> y
|
|
= ???
|
|
|
|
# Some functions return optional values:
|
|
>> Int.parse("123")
|
|
= ???
|
|
>> Int.parse("blah")
|
|
= ???
|
|
|
|
# You can check if a value exists with `if`:
|
|
n := Int.parse("123")
|
|
if n:
|
|
# Inside this condition, `n` is known to be non-none
|
|
n = add(n, 1)
|
|
>> n
|
|
= ???
|
|
|
|
# Optionals are useful for handling missing data:
|
|
name := none:Text
|
|
greeting := if name:
|
|
"Hello, $name!"
|
|
else:
|
|
"Hello, stranger!"
|
|
|
|
>> greeting
|
|
= ???
|
|
|
|
# Optional values can be converted to non-optional using `or`
|
|
>> Int.parse("blah") or 0
|
|
= ???
|
|
|
|
# They can also be converted using the `!` operator, which
|
|
# will give an error if a non-none value is encountered:
|
|
>> add(Int.parse("123")!, 1)
|
|
= ???
|
|
|
|
func add(x:Int, y:Int -> Int):
|
|
return x + y
|