included sample and extension .ML extension for Standard ML files

samples/Standard ML/Foo.ML

@@ -0,0 +1,75 @@
+
+structure LazyBase:> LAZY_BASE =
+   struct
+      type 'a lazy = unit -> 'a
+
+      exception Undefined
+
+      fun delay f = f
+
+      fun force f = f()
+
+      val undefined = fn () => raise Undefined
+   end
+
+structure LazyMemoBase:> LAZY_BASE =
+   struct 
+
+      datatype 'a susp = NotYet of unit -> 'a
+                       | Done of 'a
+
+      type 'a lazy = unit -> 'a susp ref
+
+      exception Undefined
+
+      fun delay f = 
+          let 
+             val r = ref (NotYet f)
+          in
+             fn () => r
+          end
+
+      fun force f = 
+          case f() of
+             ref (Done x) => x
+           | r as ref (NotYet f') =>
+             let
+                val a = f'()
+             in
+                r := Done a
+              ; a
+             end
+
+      val undefined = fn () => raise Undefined
+   end
+
+functor LazyFn(B: LAZY_BASE): LAZY' =
+   struct
+
+      open B
+
+      fun inject x = delay (fn () => x)
+
+      fun isUndefined x =
+          (ignore (force x)
+         ; false)
+          handle Undefined => true
+                              
+      fun toString f x = if isUndefined x then "_|_" else f (force x)
+
+      fun eqBy p (x,y) = p(force x,force y)
+      fun eq (x,y) = eqBy op= (x,y)
+      fun compare p (x,y) = p(force x,force y)
+
+      structure Ops = 
+         struct 
+            val ! = force
+            val ? = inject
+         end
+
+      fun map f x = delay (fn () => f (force x))
+
+   end
+
+structure Lazy' = LazyFn(LazyBase)
+structure LazyMemo = LazyFn(LazyMemoBase)