code / tomo-koans

   1 # Optional Types
   2 
   3 func main()
   4 
   5     # Any type can be optional, which means it will either be
   6     # `none` or have a value. You can declare an optional variable
   7     # and initialize it to `none` like this:
   8     y : Int? = none
   9 
  10     assert y == ???
  11 
  12     # Optional variables can be either `none` or a value of that type:
  13 
  14     y = 123
  15     assert y == 123
  16 
  17     # Some functions return optional values.
  18     # Int.parse(text:Text -> Int?) is a function that returns
  19     # an integer value found in the text, or `none` if no integer
  20     # is found.
  21     assert Int.parse("123") == ???
  22     assert Int.parse("blah") == ???
  23 
  24     # You can check if a value exists with `if`:
  25     n := Int.parse("123")
  26     if n
  27         # Inside this condition, `n` is known to be non-none
  28         n = add(n, 1)
  29         assert n == ???
  30 
  31     # Optionals are useful for handling missing data:
  32     name : Text? = none
  33     greeting := if name
  34         "Hello $name"
  35     else
  36         "Hello stranger"
  37 
  38     assert greeting == ???
  39 
  40     # Optional values can be converted to non-optional using `or`
  41     assert Int.parse("blah") or 0 == ???
  42 
  43     # They can also be converted using the `!` operator, which
  44     # will give an error if a non-none value is encountered:
  45     assert add(Int.parse("123")!, 1) == ???
  46 
  47     # If you index into a list, the value is optional because
  48     # you might have used an index that isn't in the list:
  49     list := [10, 20, 30]
  50     assert list[1] == 10
  51     assert list[999] == none
  52 
  53     # So, it's common to see `list[i]!` to force the value to
  54     # be non-optional or error if you made a mistake:
  55     assert list[3]! == ???
  56 
  57 func add(x:Int, y:Int -> Int)
  58     return x + y