[Haskell-cafe] Difficulty making a TH template for a monadic expression
Michael Jones
mike at proclivis.com
Tue Mar 3 16:11:40 UTC 2015
I failed to strip all the mkNames from the example. They are in the current code but will be moved inside the make functions later. So...
> $(makeCommandData ("RegTCA9535") ["INPUT_PORT_0",
> "INPUT_PORT_1",
> "OUTPUT_PORT_0",
> "OUTPUT_PORT_1",
> "POLARITY_INVERSION_PORT_0",
> "POLARITY_INVERSION_PORT_1",
> "CONFIGURATION_PORT_0",
> "CONFIGURATION_PORT_1"])
>
> (makeBitData ("TCA9535_INPUT_PORT_0_BITS") ["I0_7",
> "I0_6",
> "I0_5",
> "I0_4",
> "I0_3",
> "I0_2",
> "I0_1",
> "I0_0”])
On Mar 3, 2015, at 9:00 AM, Michael Jones <mike at proclivis.com> wrote:
> Adam,
>
> I used TH because I wanted a non-programmer to write simple statements from data sheets that generated code for a programmer. My ignorance may prove my undoing, but if I learn something by going down a rabbit hole, I can recover.
>
> I don’t need to implement g necessarily, as it is part of a larger function generating other TH code, f would be fine.
>
> The goal is to have a non-programmer write something like:
>
> $(makeCommandData (mkName "RegTCA9535") ["INPUT_PORT_0",
> "INPUT_PORT_1",
> "OUTPUT_PORT_0",
> "OUTPUT_PORT_1",
> "POLARITY_INVERSION_PORT_0",
> "POLARITY_INVERSION_PORT_1",
> "CONFIGURATION_PORT_0",
> "CONFIGURATION_PORT_1"])
>
> (makeBitData (mkName "TCA9535_INPUT_PORT_0_BITS") [mkName "I0_7",
> "I0_6",
> "I0_5",
> "I0_4",
> "I0_3",
> "I0_2",
> "I0_1",
> "I0_0”])
>
> MORE REGISTERS HERE
>
>
>
> and generate a complete API that works off a list of bits, and read/writes SMBus.
>
> I have a GSOC project posted here: http://elinux.org/Minnowboard:GSoC2015
>
> The code I am working on here is kind of starter code for that. I already have an SMBus API and impl as well on a MinnowBoardMax running Ubuntu.
>
> If any students are interested, follow the link.
>
> Mike
>
> On Mar 3, 2015, at 6:03 AM, adam vogt <vogt.adam at gmail.com> wrote:
>
>> Hi Mike,
>>
>> Is there some reason you decided to use TH, when it looks like you can write:
>>
>> f :: a -> Binary (Maybe a)
>> f con = do
>> v <- BG.getBit
>> return (do guard v; Just con)
>>
>> makeBits :: [a] -> Binary [a]
>> makeBits con = catMaybes <$> mapM f con
>>
>> and have the TH part be much smaller:
>>
>> toCons :: [Name] -> ExpQ
>> toCons = listE . map conE
>>
>> makeBits $(toCons bitNames)
>>
>>
>>
>> If you really do need to generate code, let me suggest
>>
>> combine :: [ExpQ] -> ExpQ
>> combine = foldr1 (\ a b -> [| $a >>= $b |])
>>
>> together with
>>
>> g :: Name -> ExpQ
>> g name = [| \bits -> ifM getBit ((return $(conE name) : bits) (return bits) |]
>>
>> gets you
>>
>> makeBits = combine . map g
>>
>>
>> Or you could keep the recursion explicit and write the first clause of
>> your makeBits:
>>
>> makeBits [name] = g name -- g as above
>>
>> Regards,
>> Adam
>>
>>
>> On Tue, Mar 3, 2015 at 1:05 AM, Michael Jones <mike at proclivis.com> wrote:
>>> 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
>>>
>>>
>>>
>>>
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>
>
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