diff --git a/src/Data/IsTy.hs b/src/Data/IsTy.hs
--- a/src/Data/IsTy.hs
+++ b/src/Data/IsTy.hs
@@ -1,4 +1,6 @@
 {-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE TypeFamilies, ConstraintKinds, KindSignatures
+           , FlexibleInstances, MultiParamTypeClasses #-}
 {-# OPTIONS_GHC -Wall #-}
 ----------------------------------------------------------------------
 -- |
@@ -14,8 +16,16 @@
 
 module Data.IsTy (IsTy(..)) where
 
+import GHC.Prim (Constraint)
+
 import Data.Proof.EQ ((:=:))
 
+class Yes (f :: * -> *) a
+instance Yes f a
+
 -- | Type class for typed type representations
 class IsTy ty where
-  tyEq :: ty a -> ty b -> Maybe (a :=: b)
+  type IsTyConstraint ty z :: Constraint
+  type IsTyConstraint ty z = Yes ty z
+  tyEq :: (IsTyConstraint ty a, IsTyConstraint ty b) =>
+          ty a -> ty b -> Maybe (a :=: b)
diff --git a/ty.cabal b/ty.cabal
--- a/ty.cabal
+++ b/ty.cabal
@@ -1,5 +1,5 @@
 Name:                ty
-Version:             0.1.0
+Version:             0.1.2
 Synopsis:            Typed type representations and equality proofs
 Category:            Data
 Cabal-Version:       >= 1.6
@@ -12,7 +12,7 @@
 Author:              Conal Elliott 
 Maintainer:          conal@conal.net
 Homepage:            https://github.com/conal/ty
-Copyright:           (c) 2009-2012 by Conal Elliott
+Copyright:           (c) 2009-2013 by Conal Elliott
 License:             BSD3
 License-File:        COPYING
 Stability:           experimental
@@ -24,9 +24,11 @@
 
 Library
   hs-Source-Dirs:      src
-  Build-Depends:       base<5
+  Build-Depends:       base<5, ghc-prim
   Exposed-Modules:     
                        Data.Proof.EQ
                        Data.IsTy
                        Data.Ty
   ghc-options:         -Wall
+
+ ghc-prof-options:    -prof -auto-all