Standard ML Signature Usage

gistfile1.txt

$ sml
Standard ML of New Jersey v110.67 [built: Wed Apr 10 22:39:58 2013]
- use "sigtest.sml";
[opening sigtest.sml]
[autoloading]
[library $SMLNJ-BASIS/basis.cm is stable]
[autoloading done]
signature MYLIST =
  sig
    type 'a T
    val new : 'a -> 'a T
    val add : 'a T -> 'a -> 'a T
    val len : 'a T -> int
  end
functor Test(L: sig
                  type 'a T
                  val new : 'a -> 'a T
                  val add : 'a T -> 'a -> 'a T
                  val len : 'a T -> int
                end) :
            sig val test : int L.T -> int L.T end
structure MyList :
  sig
    type 'a T = 'a list
    val new : 'a -> 'a list
    val add : 'a list -> 'a -> 'a list
    val len : 'a list -> int
  end
val it = () : unit
- structure T = Test (MyList);
structure T : sig val test : int L.T -> int L.T end
- T.test (MyList.new 100);
2
3
val it = [2,1,100] : int MyList.T
- 

sigtest.sml

signature MYLIST =
sig
  type 'a T
  val new : 'a -> 'a T
  val add : 'a T -> 'a -> 'a T
  val len : 'a T -> int
end

functor Test (L : MYLIST) =
struct
  fun test l =
  let
    val l = L.add l 1
    val _ = print (Int.toString (L.len l) ^ "\n")
    val l = L.add l 2
    val _ = print (Int.toString (L.len l) ^ "\n")
  in
    l
  end
end

structure MyList =
struct
  type 'a T = 'a list
  fun new x = [x]
  fun add xs x = x::xs
  fun len xs = length xs
end