errors

# Errors are values. A function that can fail says so in its type.

struct NotFound
  def initialize(@key : String)
  end

  def key : String
    @key
  end
end

# `Error` is a trait the compiler knows. Implementing it makes a type an
# error member wherever it appears in a union.
impl Error for NotFound
  def message : String
    "no such key: #{key}"
  end
end

STORE = {"name" => "iyi", "year" => "2026"}

def fetch(key : String) : String | NotFound
  STORE[key]? || NotFound.new(key)
end

# `!` propagates: a good value continues, an error returns from here.
def year_plus_one : Int32 | NotFound
  text = fetch("year")!
  text.to_i + 1
end

puts year_plus_one

# Handle with `case`, or fall back with `.or`, or insist with `.or_panic`.
case fetch("missing")
in String   then puts it
in NotFound then puts it.message
end

puts fetch("missing").or("default")
puts fetch("name").or_panic