Hello Haskellers, I'm currently diving into Template Haskell and I just read the original TH paper [1]. There they give the following example of a generic zip function: -- | A generic zip function. Use (e.g.,) as $(zipN 3) xs ys zs. zipN :: Int -> ExpQ zipN n = [| let zp = $(mkZip n [| zp |]) in zp |] -- | Helper function for zipN. mkZip :: Int -> ExpQ -> ExpQ mkZip n contZip = lamE pYs (caseE (tupE eYs) [m1, m2]) where (pXs, eXs) = genPEs "x" n (pXSs, eXSs) = genPEs "xs" n (pYs, eYs) = genPEs "y" n allCons = tupP $ zipWith (\x xs -> [p| $x : $xs |]) pXs pXSs m1 = match allCons continue [] m2 = match wildP stop [] continue = normalB [| $(tupE eXs) : $(appsE (contZip:eXSs))|] stop = normalB (conE '[]) -- | Generates n pattern and expression variables. genPEs :: String -> Int -> ([PatQ], [ExpQ]) genPEs x n = (pats, exps) where names = map (\k -> mkName $ x ++ show k) [1..n] (pats, exps) = (map varP names, map varE names) This works as expected, e.g., `$(zipN 3) [1..3] [4..6] [7..9]' gives [(1,4,7),(2,5,8), (3,6,9)]. However, I found this definition of passing `[| zp |]' as a helper function slightly confusing, so I tried to make it more succinct and to call zipN directly in the recursion: zipN' :: Int -> ExpQ zipN' n = lamE pYs (caseE (tupE eYs) [m1, m2]) where (pXs, eXs) = genPEs "x" n (pXSs, eXSs) = genPEs "xs" n (pYs, eYs) = genPEs "y" n allCons = tupP $ zipWith (\x xs -> [p| $x : $xs |]) pXs pXSs m1 = match allCons continue [] m2 = match wildP stop [] continue = normalB [| $(tupE eXs) : $(appsE (zipN n:eXSs)) |] stop = normalB (conE '[]) This subtle change, however, causes the compiler to diverge and to get stuck at compiling splice `$(zipN' 3) [1..3] [4..6] [7..9]'... Could anyone explain to me why the first approach works, but the 2nd small deviation does not? Is it because the compiler keeps trying to inline the recursive call to (zip N) into the template indefinitely? Are there easier (more straightforward) alternative implementations to the (imo) slightly convoluted example of zipN from the paper? Any hints are very much appreciated. Thanks, Dominik. [1] http://research.microsoft.com/~simonpj/papers/meta-haskell/