[Haskell-beginners] return IO () vs. ()

[Neil Jensen]

Hi Alex, thanks, I tried that, as well as removing the 'return' entirely,
but in both cases I ended up with the following:

    Couldn't match expected type `ISODateInt -> ISODateInt -> IO a'
           against inferred type `IO [((ISODateInt, ISODateInt), CUV)]'
    In the first argument of `(>>=)', namely
        `calcCUVs account prevMonthEnd monthEnd'
    In the first argument of `(>>)', namely
        `calcCUVs account prevMonthEnd monthEnd
       >>=
         (\ cuvs -> saveCUVs account cuvs)'
    In the expression:
            calcCUVs account prevMonthEnd monthEnd
          >>=
            (\ cuvs -> saveCUVs account cuvs)
        >>
          return ()



On Sat, May 28, 2011 at 2:50 PM, Alex Rozenshteyn <rpglover64 at gmail.com>wrote:

> The thing that I see as weird is (>>= return ()), since the right argument
> to (>>=) is supposed to be a function.
>
> Maybe you want (>>)
>
> On Sat, May 28, 2011 at 5:35 PM, Neil Jensen <neilcjensen at gmail.com>wrote:
>
>> I've been attempting to refactor some working code and running into
>> confusion about returning IO values.
>>
>> The basic sequence is to query a database, calculate some values, and then
>> store the results back in the database.
>>
>> The function which does the querying of the db and calculating results has
>> the following type signature:
>> calcCUVs :: AccountId -> IO [((ISODateInt, ISODateInt), CUV)]
>>
>> This function stores the results back into the database
>> saveCUVs :: AccountId -> [((ISODateInt, ISODateInt), CUV)] -> IO ()
>> saveCUVs account cuvs = do
>>             r' <- mapM (\x -> storeCUV (snd $ fst x) account (snd x)) cuvs
>>             return ()
>>
>>
>> I had a working variation of the below using 'do' notation, but for some
>> reason when I moved to using bind, I'm getting messed up with return values.
>>
>> processAccountCUVs :: AccountId -> ISODateInt -> ISODateInt -> IO ()
>> processAccountCUVs account prevMonthEnd monthEnd = -- do
>>                    if (prevMonthEnd == 0 && monthEnd == 0)
>>                        then calcCUVs account >>=  (\cuvs -> saveCUVs
>> account cuvs) >>= return ()
>>                        else calcCUVs account prevMonthEnd monthEnd >>=
>> (\cuvs -> saveCUVs account cuvs) >>= return ()
>>
>>
>> The compiler gives the following error message:
>>
>> Couldn't match expected type `IO ()' against inferred type `()'
>>     In the first argument of `return', namely `()'
>>     In the second argument of `(>>=)', namely `return ()'
>>     In the expression:
>>             calcCUVs account >>= (\ cuvs -> saveCUVs account cuvs)   >>=
>> return ()
>>
>>
>> I thought the last return () would correctly return us IO () as we are in
>> the IO monad... what am I missing?
>>
>> Thanks for any input you can provide.
>> Neil
>>
>>
>>
>>
>>
>>
>>
>>
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>>
>>
>
>
> --
>           Alex R
>
>
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