Re: [Haskell-beginners] Re: ghci access to .hs functions

The problem in the "all" case branch is that, although you applied the mapM-to-mapM_ modification, you didn't change the type signature of gtKys accordingly: mapM :: (a -> m b) -> m a -> m [b] mapM_ :: (a -> m b) -> m a -> m () In your case 'a' above is whatever 'fromSql' returns for your program (I can't infer it... which might explain the lengthy type signature), and 'b' is simply '()'. So 'b=()' which means the type of 'gtKys' is no longer: gtKys :: (IConnection conn) => conn -> IO [()] -- using 'mapM' gtKys :: (IConnection conn) => conn -> IO () -- using 'mapM_' Two more things to say: You should know that '()' is both a type AND a value... kind of strange. '()' designates a type of "things" (like 'Int' and 'Double' do for integers and double-precision numbers), but the kink is that the only "thing" of such type is a value known as 'unit' which is written (for good or bad) '()'. As a mental help... all functions in Control.Monad (and several other modules as well) that end in '_' "throw away" their results; meaning that they return 'unit' regardless of their action. "however, i still don't quite understand what the return is doing beyond you seem to need it in order to get things out of a monad associated function." Pan, I think It's time for you to get serious with the monads XD I can't suggest anything, 'cause I read a whole lot before my brain started to understand any of it... but maybe you should check out articles, looking for real monad-related answers to your more general questions (in particular you seem to be getting hit by "getting a value out of a monad"). Try the en.wikibooks.org/wiki/Haskell or learnyouahaskell.org and skip to the sections where do-notation and monadic computations are discussed. Just rememeber: "Don't Panic". El jue, 12-08-2010 a las 13:18 -0700, prad escribió:

On Thu, 12 Aug 2010 12:00:44 -0300 MAN wrote:

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couple of things you could be interested to know.

most definitely! i very much appreciate the help, el. thx to you too brent for clearing up the ExitCode problem

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Your main will allways be 'IO ()' , but that doesn't mean you must sparkle 'return ()' all over the place :P

well i have been specializing in random programming => just keep trying things randomly and hope it works. :D

putting return() in the "all" worked (no idea why), so i thought it must be a good thing and put it in the other two. :D

i'd also used mapM because map didn't work and i figured it had something to do with monads and M is the first letter in monad. :D

i really have to get away from this sort of thing and i'm trying to figure out the excellent stuff etugrul and kyle provided in the = vs <- thread.

now i tried taking the returns out and things are fine for "add" and "upd", but even with the changes you suggested for gtKys (mapM to mapM_) i'm getting these errors:

====== gadit.hs:30:19: Couldn't match expected type `()' against inferred type `[()]' Expected type: IO () Inferred type: IO [()] In the expression: gtKys conn In a case alternative: "all" -> gtKys conn

gadit.hs:66:4: Couldn't match expected type `[()]' against inferred type `()' Expected type: IO [()] Inferred type: IO () In the expression: mapM_ (mkPag conn) kL In the expression: do { r <- quickQuery conn "SELECT key from main" []; let kL = concat $ map (map fromSql) r; mapM_ (mkPag conn) kL } =======

it want some sort of list and i'm not providing it.

here is the code in question with the line numbers:

=======

21 main = do 22 args <- getArgs 23 let act = head args 24 conn <- connectPostgreSQL "host=localhost dbname=lohv user=pradmin" 25 case act of 26 "add" -> do 27 kV1 <- dbDef conn 28 upDbs conn (fromSql kV1) 29 "upd" -> upDbs conn (last args) 30 "all" -> gtKys conn 31 _ -> putStrLn "add, upd num, all only!!" 32 commit conn 33 disconnect conn 34 putStrLn "All Done!"

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61 -- gtKys: gets all key values in database 62 gtKys :: (IConnection conn) => conn -> IO [()] 63 gtKys conn = do 64 r <- quickQuery conn "SELECT key from main" [] 65 let kL = concat $ map (map fromSql) r 66 mapM_ (mkPag conn) kL

========

now i got to thinking about all this and realized that gtKys really shouldn't have mapM_ (mkPag conn) kL in there anyway because its job is to just get some key values not to make Pages (mkPag) in fact, i only put it in there because i couldn't figure out how to get the stuff out - as kyle says in the other thread: "once something is "inside" of a monad (IO in this case), it's very difficult, impossible, to get it out again."

so what i did is rewrite the code like this: case act of ... "all" -> do kyL <- gtKys conn mapM_ (mkPag conn) kyL

and

gtKys conn = do r <- quickQuery conn "SELECT key from main" [] return $ concat $ map (map fromSql) r

it all works now.

gtKys now has the lengthy type: gtKys :: (IConnection conn, Data.Convertible.Base.Convertible SqlValue a) => conn -> IO [a]

which i'm leaving out since it generates a scope error unless i import something else (as brent explained in the above post regarding ExitCode).

however, i still don't quite understand what the return is doing beyond you seem to need it in order to get things out of a monad associated function. whenever i have an IO () i seem to require it.

there seem to be several ways to ask functions to provide computations and require specific ways to get access to them.

-- In friendship, prad

... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners