Result Monad in Crystal Lang

1_result.cr

enum Either
  Ok
  Err
end

struct Result(T)
  getter value : T
  getter type : Either

  def initialize(@value, @type); end

  def then(&block)
    raise @value if @type.err?
    yield @value
  end

  def then(ok : Proc, err : Proc)
    return ok.call(@value) if @type.ok?
    err.call(@value)
  end

  def self.ok(val : T)
    Result(T).new(val, Either::Ok)
  end

  def self.err(val : T)
    Result(T).new(val, Either::Err)
  end

  def ok? : Bool
    @type.ok?
  end

  def err? : Bool
    @type.err?
  end
end

2_fictional_example.cr

# FICTIONAL EXAMPLE
# assuming all methods return a Result(T)
create_from_string(str).then(
  ok: ->(v : Foo){ validate_x(v) },
  err: ->(e : Exception){ control_what_happens_on_error(e) }
).then { |v|
  validate_y(v)
}.then { |v| 
  save(v)
}.then(
  ok: ->(v : Foo) { http_client_post(v) },
  err: ->(v : Exception) { maybe_even_revert_something_from_before(v) }
)