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 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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On Saturday 28 May 2011 23:35:11, Neil Jensen 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 ()

You're not using the result of mapM, so you should use mapM_ here, if the list is long or the results of storeCUV are large, it's also more efficient. saveCUVs account cuvs = mapM_ (\x -> storeCUV (snd $ fst x) account (snd x)) cuvs or, avoiding the snd and fst by taking the argument apart via pattern- matching, saveCUVs account cuvs = mapM_ (\((_,date),cuv) -> storeCUV date account cuv) cuvs

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 ()

The second argument of (>>=) must be a function, here of type (a -> IO b), ... >> return () or ... >>= return would typecheck, but the latter doesn't make any sense, since 'action >>= return' is the same as plain 'action'. The latter is only of use to finally get the right result type, which is what you're doing here.

else calcCUVs account prevMonthEnd monthEnd >>=

No, that can't be. calcCUVs takes one argument and returns an IO [...], here you pass it three.

(\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

On Sat, May 28, 2011 at 4:35 PM, Neil Jensen 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 ()

I think you might have your parenthesis in the wrong spot. Instead of

x >>= (\a -> y a) >> z

you probably want:

x >>= (\a -> y a >> z)

Does that make sense? Antoine