[Ur] Polymorphic datatype needed too late

[Benjamin Barenblat]

Consider a simple tree in which internal nodes carry structure and
leaves carry data.

  datatype tree a =
      Leaf of a
    | Node of list (tree a)

I can define a mapM operation on such a tree – it simply traverses the
tree and applies a monadic function to each leaf, collecting the
results.

  fun mapM
      [m ::: Type -> Type] (_ : monad m) [a ::: Type] [b ::: Type]
      (f : a -> m b) (tree : tree a)
      : m (tree b) =
    case tree of
        Leaf a =>
          b <- f a;
          return (Leaf b)
      | Node as =>
          bs <- List.mapM (mapM f) as;
          return (Node bs)

Now, I can create a tree and try some simple operations on it.

  val nothing : tree (transaction int) = Node []

  val main : transaction page =
    result <- mapM (fn x => x) nothing;
    return <xml></xml>

When I attempt to compile this, however, I get many ‘unsupported
expression’, ‘unsupported type constructor’, and ‘polymorphic datatype
needed too late’ errors.  You can see a complete log of the errors at
http://lpaste.net/8965946982921142272.  What do they mean?

I’m running the latest Ur/Web release, 20160213.

Thanks in advance,
Benjamin