I have a number of functions which have some arguments and produce a single result. all the arguments are in a heterogenous list and the results should update the list. this appears somewhat similar to the keyword-argument solution for functions proposed by oleg kiselyov in 2004. i would like to have a method to convert the functions f :: A -> B -> C g:: A -> D -> C -> B (for example) to f' :: argTypes -> H -> H g' :: argTypes -> H -> H where H is a heterogenous list containing A, B, C, D (and others) and argTypes is another heterogenous list containing the types of the arguments, which are used to retrieve the data from the list (with .!!.) - the result has the correct type and is updated in the list with .@@. . naively this could be done by some functins fx (recursing over the list of arguments) storing the last argument in the HList: fx :: (HList l) => op -> HCons e l -> H -> H fx op HNil uod = uod .@@. op fx op (HCons a as) uod = fx (op arg) as uod where arg = uod .!!. a this code does not compile, because HNil and HCons do not match (they are different data types, not a union type). code that compiles looks like class (HList l, HList k , HUpdateAtHNat n e k k, HType2HNat e k n , HLookupByHNat n k e ) => HCall e n f l k | l k -> n where hcall :: f -> l -> k -> k -- not general instance ( HNat n , HUpdateAtHNat n e UoD UoD, HType2HNat e UoD n , HLookupByHNat n UoD e ) => HCall e n e HNil UoD where hcall opv HNil k = hcall0 opv k instance (HCall e n f l UoD , HLookupByHNat n UoD e, HType2HNat e n UoD ) => HCall e n (e -> f) (HCons a l) UoD where hcall op (HCons a l) k = hcall (op arg) l k where arg = hLookupByHNat n k n = hType2HNat (toProxy arg) k :: n hcall0 :: ( HUpdateAtHNat n f UoD UoD, HType2HNat f H n, HCall e n f HNil UoD) => f -> H -> H hcall0 op k = hUpdateAtHNat n (const op undefined) k `asTypeOf` k where n = hType2HNat (toProxy op) k where i factored out the terminating call, which is probably wrong. i would like to use it as: showPop4 :: (HList hNil, HCall Population n (District -> Population) (hConst District hNil) UoD) => (hCons District hNil) -> UoD -> UoD showPop4 arg uod = hcall showPop2 arg uod unfortunately, calls to this function result in 'cannot deduce' compilation error; it seems that some of the intermediate types are not known. i appreciate any help! andrew