unions

# Union types: a value that is one of several types.

# `Int32 | String` holds either. Nothing is wrapped or boxed.
def parse(text : String) : Int32 | String
  n = text.to_i?
  n ? n : "not a number: #{text}"
end

# `case ... in` must name every member, so a new member in the union
# turns every incomplete `case` into a compile error.
["12", "x", "7"].each do |text|
  case parse(text)
  in Int32  then puts "number #{it * 2}"
  in String then puts "problem, #{it}"
  end
end

# `is_a?` asks, and narrows inside the `if`.
value = parse("5")
if value.is_a?(Int32)
  puts value + 1
end

# `String?` from the nil lesson is the same idea: `String | Nil`.