I’m at wits end as to how to express a monadic expression in TH. I’ll give here two ways to express a non TH version, and then a TH expression that does not quite work. It generates code that compiles, but it does not evaluate properly like the non TH version. Fundamentally, the problem is use of a recursive function using quasi quoting similar to what is in the standard Show example. Perhaps someone will have an idea on how to fix it. I have made several attempts and failed. Non TH Example A: Do notation ————————————— let r = BG.runBitGet flags (do let bits = [] v <- BG.getBit bits <- return $ if v then I1_7:bits else bits v <- BG.getBit bits <- return $ if v then I1_6:bits else bits v <- BG.getBit bits <- return $ if v then I1_5:bits else bits v <- BG.getBit bits <- return $ if v then I1_4:bits else bits v <- BG.getBit bits <- return $ if v then I1_3:bits else bits v <- BG.getBit bits <- return $ if v then I1_2:bits else bits v <- BG.getBit bits <- return $ if v then I1_1:bits else bits v <- BG.getBit bits <- return $ if v then I1_0:bits else bits return $! bits) Non TH Example B: Bind notation —————————————— let r = BG.runBitGet flags ( return [] >>= (\bits -> ifM BG.getBit (return $ I0_7:bits) (return $ bits)) >>= (\bits -> ifM BG.getBit (return $ I0_6:bits) (return $ bits)) >>= (\bits -> ifM BG.getBit (return $ I0_5:bits) (return $ bits)) >>= (\bits -> ifM BG.getBit (return $ I0_4:bits) (return $ bits)) >>= (\bits -> ifM BG.getBit (return $ I0_3:bits) (return $ bits)) >>= (\bits -> ifM BG.getBit (return $ I0_2:bits) (return $ bits)) >>= (\bits -> ifM BG.getBit (return $ I0_1:bits) (return $ bits)) >>= (\bits -> ifM BG.getBit (return $ I0_0:bits) (return $ bits))) A TH for Example B: ———————— let bitsP = varP $ mkName "bits" let bitsE = varE $ mkName "bits" let makeBits [] = [| "" |] makeBits (name:names) = [| (\bits -> ifM BG.getBit (return $ $(conE name) : $bitsE) (return $ $bitsE)) >>= $(makeBits names) |] parse <- [d| $(varP (mkName $ "parse" ++ nameBase name)) = do flags <- G.getByteString 1 let r = BG.runBitGet flags (return [] >>= $(makeBits bitNames)) case r of Left error -> fail error Right x -> return x |] This generates: parseTCA9535_INPUT_PORT_0_BITS = do {flags_0 <- Data.Binary.Strict.Get.getByteString 1; let r_1 = Data.Binary.Strict.BitGet.runBitGet flags_0 (GHC.Base.return [] GHC.Base.>>= ((\bits_2 -> Control.Conditional.ifM Data.Binary.Strict.BitGet.getBit (GHC.Base.return GHC.Base.$ (I0_7 GHC.Types.: bits)) (GHC.Base.return GHC.Base.$ bits)) GHC.Base.>>= ((\bits_3 -> Control.Conditional.ifM Data.Binary.Strict.BitGet.getBit (GHC.Base.return GHC.Base.$ (I0_6 GHC.Types.: bits)) (GHC.Base.return GHC.Base.$ bits)) GHC.Base.>>= ((\bits_4 -> Control.Conditional.ifM Data.Binary.Strict.BitGet.getBit (GHC.Base.return GHC.Base.$ (I0_5 GHC.Types.: bits)) (GHC.Base.return GHC.Base.$ bits)) GHC.Base.>>= ((\bits_5 -> Control.Conditional.ifM Data.Binary.Strict.BitGet.getBit (GHC.Base.return GHC.Base.$ (I0_4 GHC.Types.: bits)) (GHC.Base.return GHC.Base.$ bits)) GHC.Base.>>= ((\bits_6 -> Control.Conditional.ifM Data.Binary.Strict.BitGet.getBit (GHC.Base.return GHC.Base.$ (I0_3 GHC.Types.: bits)) (GHC.Base.return GHC.Base.$ bits)) GHC.Base.>>= ((\bits_7 -> Control.Conditional.ifM Data.Binary.Strict.BitGet.getBit (GHC.Base.return GHC.Base.$ (I0_2 GHC.Types.: bits)) (GHC.Base.return GHC.Base.$ bits)) GHC.Base.>>= ((\bits_8 -> Control.Conditional.ifM Data.Binary.Strict.BitGet.getBit (GHC.Base.return GHC.Base.$ (I0_1 GHC.Types.: bits)) (GHC.Base.return GHC.Base.$ bits)) GHC.Base.>>= ((\bits_9 -> Control.Conditional.ifM Data.Binary.Strict.BitGet.getBit (GHC.Base.return GHC.Base.$ (I0_0 GHC.Types.: bits)) (GHC.Base.return GHC.Base.$ bits)) GHC.Base.>>= ""))))))))); Problems with TH ———————— The problem is the () that interferes with the order of evaluation, and the termination at the end ( “” ). I’m no so worried about the termination. I can put something harmless there. The parens are the main problem. Calling a quasi quoter recursively is the cause, as it nests the evaluation. I tried things like building the bits in a list, but that does not work because the BG.getBit has to run in the BitGit monad. I know I can write a simple evaluation that just returns a list of Bools and only TH for bit names, but in the final version the size of bit fields needs to be dynamic, so I need to dynamically generate code piece by piece. I would prefer to use quasi quoting rather than build the whole thing with data types so that it is more readable. If anyone knows of a module on hackage that does something similar, perhaps you can point me to that so I can study it. Thanks…Mike