However, it seems that your particular problem can be solved with simpler means:
instance (HList a) => HListAppendArbitrary a HNil a where hAppendArbitrary a _ = a instance (HList a, HList b, HList c) => HListAppendArbitrary a (HCons b d) c where hAppendArbitrary a b = hAppend a b -- or use HCons with recursion
-- overlapping instance HList + value (2) instance (HList a, HList c) => HListAppendArbitrary a b c where hAppendArbitrary a b = HCons b a
It looks like this, doesn't it ? (Some class isntance declarations are still missing) ============= code =================================================== class (HList c) => HListAppendArbitrary a b c | a b -> c where hAppendArbitrary :: a -> b -> c -- instances HList + HList instance HListAppendArbitrary HNil HNil HNil where -- line 134 hAppendArbitrary _ _ = HNil instance HListAppendArbitrary (HCons a b) HNil (HCons a b) where hAppendArbitrary a _ = a instance HListAppendArbitrary HNil (HCons a b) (HCons a b) where hAppendArbitrary _ a = a instance HListAppendArbitrary (HCons a b) (HCons b c) (HCons a l) where hAppendArbitrary a b = hAppend a b -- instance HList + value instance HListAppendArbitrary HNil b (HCons b HNil) where -- line 143 hAppendArbitrary _ b = HCons b HNil instance HListAppendArbitrary (HCons a b) c d where hAppendArbitrary a b = hAppend a (HCons b HNil) -- instance value + HList instance HListAppendArbitrary b HNil c where hAppendArbitrary b _ = HCons b HNil instance HListAppendArbitrary a (HCons b c) d where hAppendArbitrary a b = hAppend (HCons a HNil) b ============= end code =============================================== But I'm getting the same error twice: hps-lib/HPS/Utils.hs|134| 0: || Functional dependencies conflict between instance declarations: || instance [incoherent] HListAppendArbitrary HNil HNil HNil -- Defined at hps-lib/HPS/Utils.hs|134| 0 || instance [incoherent] HListAppendArbitrary HNil b (HCons b HNil) -- Defined at hps-lib/HPS/Utils.hs|143| 0 If you compare those two lines g instance HListAppendArbitrary HNil HNil HNil where -- line 134 instance HListAppendArbitrary HNil b (HCons b HNil) where -- line 143 I see that HNil HNil -> HNil doesn't fit HNil b (thus maybe HNil) -> HCons b HNil But I don't want to omit the funtcional dependency because the resulting type should be determined by the first two parameters. As I don't know how to solve this right now I'll use HLists anywhere. Marc