diff --git a/src/libraries/turtle/check.hpp b/src/libraries/turtle/check.hpp
index 3abc59b..f1521d8 100644
--- a/src/libraries/turtle/check.hpp
+++ b/src/libraries/turtle/check.hpp
@@ -15,6 +15,8 @@
 #include "format.hpp"
 #include <boost/function.hpp>
 #include <boost/utility/enable_if.hpp>
+#include <boost/concept/assert.hpp>
+#include <boost/concept_check.hpp>
 #include <stdexcept>
 #include <ostream>
 
@@ -22,47 +24,84 @@ namespace mock
 {
 namespace detail
 {
+    template< typename Constraint, typename Actual >
+    struct ConstraintCompatible : private boost::noncopyable
+    {
+    public:
+        BOOST_CONCEPT_USAGE( ConstraintCompatible )
+        {
+            // if an error is generated by the line below it means an argument
+            // passed to 'with' was of the wrong type.
+            constraint_accepts( actual_argument_type );
+        }
+    private:
+        ConstraintCompatible( int ) {}
+        Constraint constraint_accepts;
+        Actual actual_argument_type;
+    };
+
+    template< typename Expected, typename Actual >
+    struct EqualityComparable : private boost::noncopyable
+    {
+    public:
+        BOOST_CONCEPT_USAGE( EqualityComparable )
+        {
+            boost::require_boolean_expr(
+                // if an error is generated by the line below it means an argument
+                // passed to 'with' was of the wrong type.
+                actual_argument_type == expected_argument_type
+            );
+        }
+    private:
+        EqualityComparable( int ) {}
+        Expected expected_argument_type;
+        Actual actual_argument_type;
+    };
+
     template< typename Actual >
     class check
     {
         typedef BOOST_DEDUCED_TYPENAME
-            boost::function< bool( Actual ) > functor_type;
+            boost::function< bool( Actual ) > constraint_type;
 
     public:
-        template< typename Functor >
-        explicit check( const Functor& functor,
+        template< typename Constraint >
+        explicit check( const Constraint& constraint,
             BOOST_DEDUCED_TYPENAME boost::enable_if<
-                BOOST_DEDUCED_TYPENAME detail::is_functor< Functor >
+                BOOST_DEDUCED_TYPENAME detail::is_functor< Constraint >
             >::type* = 0 )
-            : functor_( functor )
-            , desc_   ( "?" )
+            : desc_( "?" )
         {
-            if( !functor_ )
-                std::invalid_argument( "invalid functor" );
+            BOOST_CONCEPT_ASSERT(( ConstraintCompatible< Constraint, Actual > ));
+            constraint_ = constraint;
+            if( !constraint_ )
+                std::invalid_argument( "invalid constraint" );
         }
         template< typename Expected >
         explicit check( const Expected& expected,
             BOOST_DEDUCED_TYPENAME boost::disable_if<
                 BOOST_DEDUCED_TYPENAME detail::is_functor< Expected >
             >::type* = 0 )
-            : functor_( mock::equal( expected ).functor_ )
-            , desc_   ( format( expected ) )
+            : desc_( format( expected ) )
         {
-            if( !functor_ )
-                std::invalid_argument( "invalid functor" );
+            BOOST_CONCEPT_ASSERT(( EqualityComparable< Expected, Actual > ));
+            constraint_ = mock::equal( expected ).constraint_;
+            if( !constraint_ )
+                std::invalid_argument( "invalid constraint" );
         }
         template< typename Constraint >
         explicit check( const placeholder< Constraint >& ph )
-            : functor_( ph.functor_ )
-            , desc_   ( ph.desc_ )
+            : desc_( ph.desc_ )
         {
-            if( !functor_ )
-                std::invalid_argument( "invalid functor" );
+            BOOST_CONCEPT_ASSERT(( ConstraintCompatible< Constraint, Actual > ));
+            constraint_ = ph.constraint_;
+            if( !constraint_ )
+                std::invalid_argument( "invalid constraint" );
         }
 
         bool operator()( Actual actual ) const
         {
-            return functor_( actual );
+            return constraint_( actual );
         }
 
         friend std::ostream& operator<<( std::ostream& s, const check& c )
@@ -71,7 +110,7 @@ namespace detail
         }
 
     private:
-        functor_type functor_;
+        constraint_type constraint_;
         std::string desc_;
     };
 }
diff --git a/src/libraries/turtle/constraint.hpp b/src/libraries/turtle/constraint.hpp
index 507fe43..72b42d5 100644
--- a/src/libraries/turtle/constraint.hpp
+++ b/src/libraries/turtle/constraint.hpp
@@ -46,7 +46,7 @@ namespace detail
         placeholder()
             : desc_( "any" )
         {}
-        any functor_;
+        any constraint_;
         std::string desc_;
     };
     template<>
@@ -55,7 +55,7 @@ namespace detail
         placeholder()
             : desc_( "negate" )
         {}
-        negate functor_;
+        negate constraint_;
         std::string desc_;
     };
     template<>
@@ -64,7 +64,7 @@ namespace detail
         placeholder()
             : desc_( "evaluate" )
         {}
-        evaluate functor_;
+        evaluate constraint_;
         std::string desc_;
     };
 }
@@ -102,7 +102,7 @@ namespace detail
         detail::or_< detail::less< T >, detail::equal< T > > >
     less_equal( T t )
     {
-        return constraint( (less( t ) || equal( t )).functor_,
+        return constraint( (less( t ) || equal( t )).constraint_,
             "less_equal", t );
     }
 
@@ -111,7 +111,7 @@ namespace detail
         detail::or_< detail::greater< T >, detail::equal< T > > >
     greater_equal( T t )
     {
-        return constraint( (greater( t ) || equal( t )).functor_,
+        return constraint( (greater( t ) || equal( t )).constraint_,
             "greater_equal", t );
     }
 
diff --git a/src/libraries/turtle/placeholder.hpp b/src/libraries/turtle/placeholder.hpp
index b15548b..789d761 100644
--- a/src/libraries/turtle/placeholder.hpp
+++ b/src/libraries/turtle/placeholder.hpp
@@ -15,14 +15,14 @@ namespace mock
 {
 namespace detail
 {
-    template< typename Functor >
+    template< typename Constraint >
     struct placeholder
     {
-        placeholder( const Functor& f, const std::string& desc )
-            : functor_( f )
-            , desc_   ( desc )
+        placeholder( const Constraint& c, const std::string& desc )
+            : constraint_( c )
+            , desc_      ( desc )
         {}
-        Functor functor_;
+        Constraint constraint_;
         std::string desc_;
     };
 
@@ -84,7 +84,7 @@ namespace detail
                     const placeholder< F2 >& rhs )
     {
         return placeholder< or_< F1, F2 > >(
-            or_< F1, F2 >( lhs.functor_, rhs.functor_ ),
+            or_< F1, F2 >( lhs.constraint_, rhs.constraint_ ),
             "(" + lhs.desc_ + " || " + rhs.desc_ + ")" );
     }
 
@@ -94,7 +94,7 @@ namespace detail
                     const placeholder< F2 >& rhs )
     {
         return placeholder< and_< F1, F2 > >(
-            and_< F1, F2 >( lhs.functor_, rhs.functor_ ),
+            and_< F1, F2 >( lhs.constraint_, rhs.constraint_ ),
             "(" + lhs.desc_ + " && " + rhs.desc_ + ")" );
     }
 
@@ -103,7 +103,7 @@ namespace detail
         operator!( const placeholder< F >& ph )
     {
         return placeholder< not_< F > >(
-            not_< F >( ph.functor_ ), "! " + ph.desc_ );
+            not_< F >( ph.constraint_ ), "! " + ph.desc_ );
     }
 }
 }
diff --git a/src/tests/turtle_test/constraint_test.cpp b/src/tests/turtle_test/constraint_test.cpp
index 466e423..8366af3 100644
--- a/src/tests/turtle_test/constraint_test.cpp
+++ b/src/tests/turtle_test/constraint_test.cpp
@@ -48,21 +48,21 @@ BOOST_AUTO_TEST_CASE( same )
     int i = 0;
     int j = 0;
     BOOST_CHECK_EQUAL( i, j );
-    BOOST_CHECK( ! mock::same( i ).functor_( j ) );
-    BOOST_CHECK( mock::same( i ).functor_( i ) );
+    BOOST_CHECK( ! mock::same( i ).constraint_( j ) );
+    BOOST_CHECK( mock::same( i ).constraint_( i ) );
 }
 
 BOOST_AUTO_TEST_CASE( assign )
 {
     {
         int i = 0;
-        BOOST_CHECK( mock::assign( 3 ).functor_( i ) );
+        BOOST_CHECK( mock::assign( 3 ).constraint_( i ) );
         BOOST_CHECK_EQUAL( 3, i );
     }
     {
         const int* i = 0;
         const int j = 1;
-        BOOST_CHECK( mock::assign( &j ).functor_( i ) );
+        BOOST_CHECK( mock::assign( &j ).constraint_( i ) );
         BOOST_CHECK_EQUAL( i, &j );
     }
 }
@@ -72,49 +72,49 @@ BOOST_AUTO_TEST_CASE( retrieve )
     {
         int i = 0;
         const int j = 1;
-        BOOST_CHECK( mock::retrieve( i ).functor_( j ) );
+        BOOST_CHECK( mock::retrieve( i ).constraint_( j ) );
         BOOST_CHECK_EQUAL( i, j );
     }
     {
         int* i = 0;
         int j = 1;
-        BOOST_CHECK( mock::retrieve( i ).functor_( &j ) );
+        BOOST_CHECK( mock::retrieve( i ).constraint_( &j ) );
         BOOST_CHECK_EQUAL( i, &j );
     }
     {
         const int* i = 0;
         const int j = 1;
-        BOOST_CHECK( mock::retrieve( i ).functor_( j ) );
+        BOOST_CHECK( mock::retrieve( i ).constraint_( j ) );
         BOOST_CHECK_EQUAL( i, &j );
     }
     {
         const int* i = 0;
         int j = 1;
-        BOOST_CHECK( mock::retrieve( i ).functor_( j ) );
+        BOOST_CHECK( mock::retrieve( i ).constraint_( j ) );
         BOOST_CHECK_EQUAL( i, &j );
     }
     {
         int* i = 0;
         int j = 1;
-        BOOST_CHECK( mock::retrieve( i ).functor_( j ) );
+        BOOST_CHECK( mock::retrieve( i ).constraint_( j ) );
         BOOST_CHECK_EQUAL( i, &j );
     }
     {
         const int* i = 0;
         const int j = 1;
-        BOOST_CHECK( mock::retrieve( i ).functor_( j ) );
+        BOOST_CHECK( mock::retrieve( i ).constraint_( j ) );
         BOOST_CHECK_EQUAL( i, &j );
     }
     {
         int** i = 0;
         int* j = 0;
-        BOOST_CHECK( mock::retrieve( i ).functor_( j ) );
+        BOOST_CHECK( mock::retrieve( i ).constraint_( j ) );
         BOOST_CHECK_EQUAL( i, &j );
     }
     {
         const int** i = 0;
         const int* j = 0;
-        BOOST_CHECK( mock::retrieve( i ).functor_( j ) );
+        BOOST_CHECK( mock::retrieve( i ).constraint_( j ) );
         BOOST_CHECK_EQUAL( i, &j );
     }
 }
@@ -137,15 +137,15 @@ BOOST_AUTO_TEST_CASE( retrieve_uses_assignment_operator )
 {
     B b;
     const A a = A();
-    mock::retrieve( b ).functor_( a );
+    mock::retrieve( b ).constraint_( a );
 }
 
 BOOST_AUTO_TEST_CASE( negate )
 {
     int* i = 0;
     int j;
-    BOOST_CHECK( mock::negate.functor_( i ) );
-    BOOST_CHECK( ! mock::negate.functor_( &j ) );
+    BOOST_CHECK( mock::negate.constraint_( i ) );
+    BOOST_CHECK( ! mock::negate.constraint_( &j ) );
 }
 
 namespace
@@ -162,12 +162,12 @@ namespace
 
 BOOST_AUTO_TEST_CASE( evaluate )
 {
-    BOOST_CHECK( mock::evaluate.functor_( &return_true ) );
-    BOOST_CHECK( ! mock::evaluate.functor_( &return_false ) );
+    BOOST_CHECK( mock::evaluate.constraint_( &return_true ) );
+    BOOST_CHECK( ! mock::evaluate.constraint_( &return_false ) );
 }
 
 BOOST_AUTO_TEST_CASE( contain )
 {
-    BOOST_CHECK( mock::contain( "string" ).functor_( "this is a string" ) );
-    BOOST_CHECK( ! mock::contain( "not found" ).functor_( "this is a string" ) );
+    BOOST_CHECK( mock::contain( "string" ).constraint_( "this is a string" ) );
+    BOOST_CHECK( ! mock::contain( "not found" ).constraint_( "this is a string" ) );
 }