[CakeML] Polymorphic type annotation

[Mário Pereira]

Dear list,

I am currently running some experiences around the CakeML compiler.

I ran across some difficulties when trying to annotate function 
arguments with polymorphic types. Take for instance the CPS version of a 
function that computes the height of a binary tree:

datatype 'a tree = Leaf | Node of 'a tree * 'a * 'a tree

fun height t (k: int -> int) =
   case t of
     Leaf => k 0
   | Node l _ r =>
       height l (fn hl =>
       height r (fn hr => 1 + max hl hr))

This program is accepted by CakeML. However, I would like to write the 
following more general version of it:

fun height t (k: int -> 'a) =
   case t of
     Leaf => k 0
   | Node l _ r =>
       height l (fn hl =>
       height r (fn hr => 1 + max hl hr))

which gives me "### ERROR: type error Bad type annotation in pattern".  
I see the error is not related to the arrow type, as the polymorphic 
identity function 'fun id (x: 'a) = x' is also rejected.

Am I doing something wrong with the syntax of type annotations ?

Best regards
--
Mário Pereira