I'm trying to learn more about HList. While setting up a small test application I got the following error.. ghc(i) tells me to add -fallow-incoherent-instances but doesn't recognize it ? Is this a bug? marc@localhost /pr/haskell/test/HList $ ghci -package HList HListTestMain.hs ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Loading package HList-0.1 ... linking ... done. [1 of 2] Compiling HListTest.HList ( HListTest/HList.hs, interpreted ) HListTest/HList.hs:12:11: Overlapping instances for HOccursMany (Maybe Int) (HCons t (HCons t1 (HCons [Char] (HCons (Maybe t2) HNil)))) arising from use of `hOccursMany' at HListTest/HList.hs:12:11-24 Matching instances: instance [overlap ok] (HOccursMany e l, HList l) => HOccursMany e (HCons e' l) -- Defined in HOccurs instance [overlap ok] (HOccursMany e l, HList l) => HOccursMany e (HCons e l) -- Defined in HOccurs (The choice depends on the instantiation of `t, t1, t2' Use -fallow-incoherent-instances to use the first choice above) In the expression: hOccursMany l1 In the second argument of `($)', namely `(hOccursMany l1 :: [Maybe Int])' In the expression: print $ (hOccursMany l1 :: [Maybe Int]) Failed, modules loaded: none. Prelude> Leaving GHCi. marc@localhost /pr/haskell/test/HList $ cat HListTest/HList.hs {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} {-# OPTIONS -fallow-incoherent-instances #-} -- <<<<<<<<<<<< given here module HListTest.HList where import HList l1 = 2 .*. 3 .*. "abc" .*. (Just 4) .*. HNil hlistTest = do print $ 2 .*. HNil print $ (hOccursMany l1 :: [Maybe Int]) marc@localhost /pr/haskell/test/HList $ Marc