Standard ML functor example

gistfile1.txt

signature FOO =
sig
  type 'a foo
  val f : 'a -> 'a foo
end

signature BAR =
sig
  structure Foo : FOO
  val p : 'a -> bool
end

functor F (B : BAR) : FOO =
struct
  type 'a foo = 'a B.Foo.foo
  fun f x = if B.p x
            then B.Foo.f x
            else raise Domain
end

structure SomeFoo =
struct
  type 'a foo = 'a * 'a
  fun f x = (x, x)
end

structure SomeFooP = F (struct structure Foo = SomeFoo
			       fun p x = x <> 42 end)