-- <PRE>
-- This file was inspired from our development of
-- <A HREF="SimpleTags.prg">SimpleTags.prg</A>. We replace
-- Tag' and Label' by the builtin Tag and Label type.


-- A Shape is a list or record structure in which
-- we use the built in Tag as labels.
-- The built in Tag types are constructed by using
-- the backquote on a variable like `a

data Shape:: *1 where
  E :: Shape
  P :: Tag ~> *0 ~> Shape ~> Shape
    deriving Record(s)


-- A count is a natural number, which is indexed
-- by shapes and types.

data Count:: Shape ~> Tag ~> *0 ~> *0 where
  Zero:: Count (P a t b) a t
  Succ:: Count sh nm ty -> (Count (P a ty2 sh)) nm ty
    deriving Nat(c)

-- A Term is indexed by a Shape which determines what
-- variables are used in the term, and what type those
-- variables have.

data Term:: Shape ~> *0 ~> *0 where
  Iconst :: Int -> Term sh Int
  Var:: Label a -> Count i a ty -> Term i ty
  Pair:: Term sh a -> Term sh b -> Term sh (a,b)
  Let:: Label a -> Term sh x -> Term (P a x sh) s -> Term sh s