type-level maps and maybes

[hs-rrdtool.git] / Data / HList / Prelude.hs

diff --git a/Data/HList/Prelude.hs b/Data/HList/Prelude.hs
index 3a46616662132331d8479a57ee70692cf9eed6ef..d50ad25e081c6e129442081d239f29b244f35543 100644 (file)
--- a/Data/HList/Prelude.hs
+++ b/Data/HList/Prelude.hs
@@ -342,4 +342,14 @@ instance TypeCast'' t a b => TypeCast' t a b
 instance TypeCast'' () a a
     where
       typeCast'' _ x = x
+
+
+class MemberT e l where
+    type Member e l
+
+instance MemberT e Nil where
+    type Member e Nil = False
+
+instance MemberT e (Cons e' l) where
+    type Member e (Cons e' l) = TypeEq e e' b => b
 -}
\ No newline at end of file