Question

I have a function too complicated for me to be explicit about what the function type should be. I’m trying to get GHC to agree that what I expect, is what it expects. First, the function, what I think it should be doing. Then, where the confusion comes in.

flagScheduled ((Left (MkUFD day)):rest) = do
   match <- runDB $ selectList [TestStartDate ==. Just day,
                                TestStatus /<-. [Passed,Failed]] []
   case (L.null match) of
      True -> do
               processedDays <- ([(Left $ MkUFD day)] :) <$> flagScheduled rest
               return processedDays
      False -> do
                let flaggedRange = (calcFlagged match)
                    product      = (testFirmware . snd . P.head) match
                processedDays <- (flagScheduled'
                                  ([(Left $ MkUFD day)] ++
                                   (L.take flaggedRange) rest) (show product) :) <$>
                                   (flagScheduled . L.drop flaggedRange) rest
                return processedDays
flagScheduled ((Right day):rest) = do
     processedDays <- ((Right $ day):) <$> flagScheduled rest
     return processedDays
flagScheduled _ = return []

calcFlagged (( _ ,(Test _ _ (Just startDate) (Just endDate) _ _ )) : rest) =
  fromIntegral $ C.diffDays endDate startDate

flagScheduled' toBeFlagged product =
   L.map (flagIt product) toBeFlagged
    where flagIt product (Left (MkUFD day)) = Right $
                                              MkCal $
                                              Right $
                                              MkUAD $
                                              Left  $
                                               MkSDay day
                                                      (read product :: Product)
                                                      Reserved

The idea is I start with a [Either UnFlaggedDay CalendarDay]
I iterate through the list transforming some of the UnFlaggedDays into CalendarDays.
Other functions will transform the rest of the UnFlaggedDays. Below I define the types I am working with.

newtype AvailableDay = MkAD (Text, C.Day)
                          deriving (Show, Eq)

newtype UnAvailableDay = MkUAD (Either ScheduledDay Out_Of_Office)
                             deriving Show

data ScheduledDay = MkSDay C.Day Product ScheduledState
                       deriving Show

newtype ReservedDay = MkRDay (C.Day,Product)
                                 deriving (Ord,Show,Eq,Read)

newtype ASAPDay = MkADay (C.Day,Product)
                            deriving (Ord,Show,Eq,Read)

newtype UnFlaggedDay = MkUFD C.Day

newtype CalendarDay = MkCal (Either AvailableDay UnAvailableDay)
                               deriving Show

So here is the problem, when I compile I get this error, which is not in itself confusing.

    Utils/BuildDateList.hs:173:44:
        Couldn't match expected type `Either a0 b0'
                    with actual type `[Either UnFlaggedDay CalendarDay]'
        Expected type: GGHandler sub0 master0 monad0 [Either a0 b0]
          Actual type: GGHandler
                         sub0 master0 monad0 [[Either UnFlaggedDay CalendarDay]]
        In the return type of a call of `flagScheduled'
        In the second argument of `(<$>)', namely `flagScheduled rest'

Okay fine, it looks like all I need to do is apply a well-placed concat and I can make the actual type GGHandler
sub0 master0 monad0 [[Either UnFlaggedDay CalendarDay]]
match the expected type GGHandler
sub0 master0 monad0 [[Either UnFlaggedDay CalendarDay]]

But wait, not that simple. Here’s one attempt of many, and no matter where I place the concat it seems to lead to the same error.

   Utils/BuildDateList.hs:164:16:
       Couldn't match expected type `[Either UnFlaggedDay b0]'
                   with actual type `Either UnFlaggedDay b0'
       Expected type: GGHandler
                        sub0 master0 monad0 [[Either UnFlaggedDay b0]]
         Actual type: GGHandler
                        sub0 master0 monad0 [Either UnFlaggedDay b0]
       In the expression: return $ P.concat processedDays
       In the expression:
         do { processedDays <- ([(Left $ MkUFD day)] :)
                             <$>
                               flagScheduled rest;
                return $ P.concat processedDays }

Did you see what happened there? Here are the changes I made. I passed processedDays to concat before passing it to return.

flagScheduled ((Left (MkUFD day)):rest) = do
   match <- runDB $ selectList [TestStartDate ==. Just day,
                                TestStatus /<-. [Passed,Failed]] []
   case (L.null match) of
      True -> do
               processedDays <- ([(Left $ MkUFD day)] :) <$> flagScheduled rest
               return $ P.concat processedDays
      False -> do
                let flaggedRange = (calcFlagged match)
                    product      = (testFirmware . snd . P.head) match
                processedDays <- (flagScheduled'
                                  ([(Left $ MkUFD day)] ++
                                   (L.take flaggedRange) rest) (show product) :) <$>
                                   (flagScheduled . L.drop flaggedRange) rest
                return $ P.concat processedDays
flagScheduled ((Right day):rest) = do
     processedDays <- ((Right $ day):) <$> flagScheduled rest
     return $ P.concat processedDays
flagScheduled _ = return []

So the fact that what looks like a straight-forward change that isn’t, indicates to me that I don’t really understand what the problem is. Any ideas?

Update: I made the changes Daniel suggested, but got this error:

Utils/BuildDateList.hs:169:37:
    Couldn't match expected type `[Either UnFlaggedDay t0]'
                with actual type `Either UnFlaggedDay b0'
    In the first argument of `(++)', namely `(Left $ MkUFD day)'
    In the first argument of `flagScheduled'', namely
      `((Left $ MkUFD day) ++ (P.take flaggedRange) rest)'
    In the first argument of `(:)', namely
      `flagScheduled'
         ((Left $ MkUFD day) ++ (P.take flaggedRange) rest) (show product)'

Update: This problem has been solved, only to reveal other (similar) problems. I’m going to take the advice given here to move forward with that.

Answer 1

The first suspect:

   case (L.null match) of
      True -> do
           processedDays <- ([(Left $ MkUFD day)] :) <$> flagScheduled rest
           return processedDays

Maybe that should read

   case (L.null match) of
      True -> do
           processedDays <- ((Left $ MkUFD day) :) <$> flagScheduled rest
           return processedDays

? Oh, and start with writing type signatures. That often creates better error messages.