nil

# nil is a value, and the type says whether it can appear.

# `String?` is `String | Nil`: it may hold a string, or nothing.
def lookup(id : Int32) : String?
  id == 1 ? "Ada" : nil
end

found = lookup(1)
missing = lookup(2)

puts found
puts missing

# Before you use a maybe-nil value, check it. Inside the `if`, the
# compiler knows it is a String.
if found
  puts found.upcase
end

# Or give a default with `||`.
puts missing || "nobody"

# Calling a String method on a `String?` without checking does not
# compile. That is the whole point: a nil can never surprise you at run time.