[Haskell-cafe] Difficulty making a TH template for a monadic expression

[Michael Jones]

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