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

[Tom Ellis]

On Thu, Apr 16, 2015 at 10:57:41AM +1000, Jeff wrote:
>       ( \pl' -> let pld = P.payloadData pl' in
>                  if testBit mdm ( fromEnum D.GPS )
>                    then
>                      parseDeviceGPSData >>=
>                      ( \s -> return ( pl' { P.payloadData = pld { P.gpsData = Just s } } ) )
>                    else
>                      return pl' ) >>=
>       ( \pl' -> let pld = P.payloadData pl' in
>                  if testBit mdm ( fromEnum D.GSM )
>                    then
>                      parseDeviceGSMData >>=
>                      ( \s -> return ( pl' { P.payloadData = pld { P.gsmData = Just s } } ) )
>                    else
>                      return pl' ) >>=
>       ( \pl' -> let pld = P.payloadData pl' in
>                  if testBit mdm ( fromEnum D.COT )
>                    then
>                      parseDeviceCOTData >>=
>                      ( \s -> return ( pl' { P.payloadData = pld { P.cotData = Just s } } ) )

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.

Tom