[Haskell-cafe] Advice needed on how to improve some code

[Jeff]

Thanks Sylvain,

I had a suspicion that Applicative might be applicable  (  ;-) )
I like Tom Ellis’ suggestions but I might try this out.

Jeff

> On 16 Apr 2015, at 6:24 pm, Sylvain Henry <hsyl20 at gmail.com> wrote:
> 
> I don't think you need to update record fields of a blank record: you
> can create the record using applicative operators instead. Something
> like:
> 
> parseDevicePLData :: Bool -> Get PayloadData
> parseDevicePLData hasEv = do
>   rawEvId <- if hasEv then getWord8 else return 0 -- I guessed the 0 value
>   let evId           = toEnum (fromIntegral rawEvId .&. 0x7f)
>   let statusFlag = testBit rawEvId 7
>   mask <- getWord16be
> 
>  let parseMaybe e p = if testBit mask (fromEnum e) then Just <$> p
> else return Nothing
> 
>  DevicePL hasEv statusFlag evId mask
>    <$> parseMaybe D.GPS parseDeviceGPSData
>    <*> parseMaybe D.GSM parseDeviceGSMData
>    <*> parseMaybe D.COT parseDeviceCotData
>    <*> ...
> 
> parseDevicePL :: Bool -> Get Payload
> parseDevicePL hasEv = do
>   ts <- parseTimestamp
>   P.Payload "" (Just ts)  <$> parseDevicePLData hasEv
> 
> Then you can lift these "parsers" into you Parser monad only when you need it.
> 
> -- Sylvain
> 
> 2015-04-16 8:37 GMT+02:00 Jeff <jeff at datalinktech.com.au>:
>> Thanks Tom, David and Claude for your replies.
>> 
>> 
>> 
>>> On 16 Apr 2015, at 4:03 pm, Tom Ellis <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
>>> 
>>> The first thing you should do is define
>>> 
>>>   parseDeviceGPSDataOf constructor parser setField =
>>>      ( \pl' -> let pld = P.payloadData pl' in
>>>                 if testBit mdm ( fromEnum constructor )
>>>                   then
>>>                     parser >>=
>>>                     ( \s -> return ( pl' { P.payloadData = setField pld (Just s) } } ) )
>>>                   else
>>>                     return pl' )
>>> 
>>> and your chain of binds will become
>>> 
>>>   setgpsData pld = pld { P.gpsData = Just s }
>>>   ...
>>> 
>>>   parseDeviceDataOf D.GPS parseDeviceGPSData setgpsData >>=
>>>   parseDeviceDataOf D.GSM parseDeviceDSMData setgsmData >>=
>>>   parseDeviceDataOf D.COT parseDeviceCOTData setcotData >>=
>>>   ...
>>> 
>>> Then I would probably write
>>> 
>>>   deviceSpecs = [ (D.GPS, parseDeviceGPSData, setgpsData)
>>>                 , (D.GSM, parseDeviceDSMData, setgsmData)
>>>                 , (D.COT, parseDeviceCOTData, setcotData) ]
>>> 
>>> and turn the chain of binds into a fold.
>>> 
>> 
>> 
>> I’ll do as you have suggested Tom.  Thanks.
>> 
>> Jeff
>> 
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