Surely I've overseen a small point. But I can't see it: Why doesn't ghc recognize taht elc_ (class decl), elc (instance decl) and elc_1 are the same type To help you find the connecting pieces faster I've marked them with ## ## Marc data PT a b = PT b -- phantom type containing state a and the result b class AddEl el_ el2_ elc_ where addEl :: el_ -> elc -> el2_ -- el, child -- ========== adding sub elements (tags) ============================= class AddElT est el_ estc ##elc_## est2 el2_ | estc est -> est2 , est est2 estc el2_ -> el_ , est est2 estc el2_ -> elc_ where addElT :: PT est el_ -> PT estc ##elc_## -> PT est2 el2_ -- first child ? attrs ok? instance ( AddEl el el2 ##elc## <-- this is not recognized because elc != elc_1 , Consume st (Elem celType) st' , DetermineElAddEl (NYV (Element elType AttrsOk st HFalse)) el (Valid celType) elc (NYV (Element elType AttrsOk st' HTrue)) el2 ) => AddElT (NYV (Element elType AttrsOk st HFalse)) el (Valid celType) ##elc## <-- elc_ from class declaration (NYV (Element elType AttrsOk st' HTrue)) el2 where addElT (PT t) (PT ##c##) = PT $ addEl t c -- <<<<<<<<< line 435, c should have type elc, not elc_1 src/Text/XML/Validated/Types.hs|435 col 32 error| || Could not deduce (AddEl el el2 elc_1) || from the context (AddElT || (NYV (Element elType AttrsOk st HFalse)) || el || (Valid celType) || elc || (NYV (Element elType AttrsOk st' HTrue)) || el2, || AddEl el el2 elc, || Consume st (Elem celType) st', || DetermineElAddEl || (NYV (Element elType AttrsOk st HFalse)) || el || (Valid celType) || elc || (NYV (Element elType AttrsOk st' HTrue)) || el2) || arising from a use of `addEl' || at src/Text/XML/Validated/Types.hs:435:32-40 || Possible fix: || add (AddEl el el2 elc_1) to the context of the instance declaration || In the second argument of `($)', namely `addEl t c' || In the expression: PT $ addEl t c || In the definition of `addElT': || addElT (PT t) (PT c) = PT $ addEl t c