Hi all, I'm just getting my feet wet with template haskell, and I tried to write a tmap function which maps a function over the ith component of an n-tuple (which uses a slightly different approach than the given version on the TH wiki): -- | Selects the ith component of an n-tuple tsel :: Int -> Int -> ExpQ -- n-tuple a -> a tsel i n = [| \t -> $(caseE [| t |] [alt]) |] where alt = match (tupP pats) body [] pats = map varP xs xs = [ mkName ("x" ++ show k) | k <- [1..n] ] body = normalB . varE $ xs !! (i-1) -- | Maps a function over the ith component of an n-tuple tmap :: Int -> Int -> ExpQ -- :: (a -> b) -> n-tuple -> n-tuple tmap i n = do f <- newName "f" t <- newName "t" lamE [varP f, varP t] $ [| let prefix = map extract [1..(i-1)] new = $f ($(tsel i n) $t) suffix = map extract [(i+1)..n] extract k = $(tsel k n) t in tupE $ prefix ++ [new] ++ suffix |] However, this code results in the following error: Sandbox.hs:26:29: Stage error: ‘k’ is bound at stage 2 but used at stage 1 … In the splice: $(tsel k n) In the Template Haskell quotation [| let prefix = map extract [1 .. (i - 1)] new = $f ($(tsel i n) ($t)) suffix = map extract [(i + 1) .. n] extract k = $(tsel k n) $t in tupE $ prefix ++ [new] ++ suffix |] Compilation failed. Could anyone explain to me what stage 2 and stage 1 refer to, and further, what the logical flaw in the above snippet is? What exactly is wrong with line `extract k = $(tsel k n) $t' ? Thanks! Dominik.