datatype pair a b = Pair of a * b structure M : sig class default val get : t ::: Type -> default t -> t val string_default : default string val int_default : default int val option_default : t ::: Type -> default t -> default (option t) val pair_default : a ::: Type -> b ::: Type -> default a -> default b -> default (pair a b) end = struct class default t = t fun get (t ::: Type) (x : t) = x val string_default = "Hi" val int_default = 0 fun option_default (t ::: Type) (x : t) = Some x fun pair_default (a ::: Type) (b ::: Type) (x : a) (y : b) = Pair (x, y) end open M fun default (t ::: Type) (_ : default t) : t = get val hi : string = default val zero : int = default val some_zero : option int = default val hi_zero : pair string int = default fun frob (t ::: Type) (_ : default t) : t = default val hi_again : string = frob val zero_again : int = frob fun show_option (t ::: Type) (_ : show t) : show (option t) = mkShow (fn x => case x of None => "None" | Some y => show y) fun show_pair (a ::: Type) (b ::: Type) (_ : show a) (_ : show b) : show (pair a b) = mkShow (fn x => case x of Pair (y, z) => "(" ^ show y ^ "," ^ show z ^ ")") fun main () : transaction page = return {[hi_again]}, {[zero_again]}, {[some_zero]}, {[hi_zero]}