Well I don't just want to give you the answer, but this code finds a subsequence in a list... This is from the HList library, and it looks for an ordered subtype ... but it also does what you want (it uses a four valued modal logic, which returns AllFalse, if list a is not a subtype (in-order) of b, (if the types are different there is no point in comparing values) or it returns SomeTrue, which has a value of either True or False depending on whether the list is an ordered sublist of values. -------------------------------------------------------------------- -- test for list a containing list b (elements of b occur in-order, but may -- be interleaved with other elements. class (HList r,HList s) => HContains' r s where hContains' :: r -> s -> Bool instance HContains' HNil HNil where hContains' _ _ = True instance HList r => HContains' (HCons a r) HNil where hContains' _ _ = True instance HList r => HContains' HNil (HCons a r) where hContains' _ _ = False instance (HContains' r (HCons a s),HContains' r s,Eq a) => HContains' (HCons a r) (HCons a s) where hContains' (HCons a r) s@(HCons a' s') = if a == a' then hContains' r s' else hContains' r s instance HContains' r (HCons b s) => HContains' (HCons a r) (HCons b s) where hContains' (HCons _ r) s = hContains' r s class (HList r,HList s,Logic.MBool t) => HContains'' r s t | r s -> t where hContains'' :: r -> s -> t instance HContains'' HNil HNil Logic.AllTrue where hContains'' _ _ = Logic.AllTrue instance HList r => HContains'' (HCons a r) HNil Logic.AllTrue where hContains'' _ _ = Logic.AllTrue instance HList r => HContains'' HNil (HCons a r) Logic.AllFalse where hContains'' _ _ = Logic.AllFalse instance HContains'' r s t => HContains'' (HCons a r) (HCons a s) t where hContains'' (_ :: HCons a r) (_ :: HCons b s) = hContains'' (undefined :: r) (undefined :: s) instance HContains'' r s t => HContains'' (HCons a r) s t where hContains'' (_ :: HCons a r) s = hContains'' (undefined :: r) s class (HContains' r s,Logic.Modal t) => HContains r s t | r s -> t where hContains :: r -> s -> t instance HContains' r HNil => HContains r HNil Logic.AllTrue where hContains _ _ = Logic.AllTrue instance (HContains'' r (HCons b s) t',Logic.And t' Logic.SomeTrue t,HContains' r (HCons b s)) => HContains r (HCons b s) t where hContains r s = hContains'' r s `Logic.and` Logic.toSomeTrue (hContains' r s) ---------------------------------------------------------------------------- Keean.