The type annotation of Cat is based on the idea that every program, or subprogram, is a transition from one stack to another. For example the following is the type annotation for several of the atomic programs of Cat (the primitives if you prefer):

  + : (int int) -> (int)
  - : (int int) -> (int)
  and : (bool bool) -> (bool)
  > : (int int) -> (bool)
  pop : (any) -> ()
  dup : (any) -> (any any)

  rnd : () ~> (int)
  write : (any) ~> (any)  

Cat now also supports scopes. A namespace is just a named scope. This addition makes it much easier to write non-trivial software using Cat.

Here is what some sample programs look like in the upcoming Cat:

define clear : (any*) ~> ()
{
  [pop] [is_stack_empty not] while
}

define simple_test : () -> (int)
{
  0 1 +
}

define nested_test : () -> (int)
{
  define f : () -> (int)
  {
    1
  }

  define plus_one : (int) -> (int)
  {
    1 +
  }

  f plus_one
}

namespace X
{
  define f : (int) -> (int)
  {
    2 +
  }
}

namespace Y
{
  define f : (int) -> (int)
  {
    3 +
  }
}

define main : () ~> ()
{
  "expect 1" write simple_test write clear
  "expect 2" write nested_test write clear
  "expect 3" write 1 X.f write clear
  "expect 4" write 1 Y.f write clear
}