{-# OPTIONS 
 -XMultiParamTypeClasses
 -XFlexibleInstances 
 -XOverlappingInstances
 #-}

-- A type-level environment represented as a typeclass with a single
-- opertion that references a value.
class Ref e v where ref :: e -> v

-- Environments are represented by nested tuples.  Variables are
-- indexed by type.  The first matching type is returned, so shadowing
-- (multiple SSA instances of a register) is possible.  This uses
-- OverlappingInstances to allow ``pattern mathching on types''.
instance              Ref (v,e') v  where ref (v,e) = v      -- deref
instance Ref e' v' => Ref (v,e') v' where ref (v,e) = ref e  -- recurse


-- Example
newtype R0 a = R0 a deriving Show
newtype R1 a = R1 a deriving Show
newtype R2 a = R2 a deriving Show

type Env1 a = (R1 a, (R0 a, (R0 a, ())))
env1 = (R1 1, (R0 2, (R0 3, ()))) :: Env1 Integer


-- Variable lookup then happens by type coercion:
refR1 = ref env1 :: R1 Integer
refR0 = ref env1 :: R0 Integer

----- The following fails to typecheck
-- refR2 = ref env1 :: R2 Integer