Re: problems figuring out what the type system is telling me
On Fri, Jun 07, 2002 at 09:54:16PM -0700, Hal Daume III wrote: > see http://haskell.org/wiki/wiki?ThatAnnoyingIoType and > http://haskell.org/wiki/wiki?UsingIo my apologies. please assume that i am really stupid. i have already read those two. i have also read what the hell are monads and monads for the working haskell programmer. i still do not get what i am doing wrong. getDepends :: String -> [String] getDepends p = do handle <- openFile (portsDir ++ p ++ "/+CONTENTS") ReadMode fetchDepends handle to my brain this takes a string, concatenates it with portsDir and "+/CONTENTS", and passes it to openfile. openFile then returns a handle and this handle and passes it to fetchDepends. getDepends will return whatever fetchDepends returns, assuming openFile succeeds. however ghc says Phoebe.hs:19: Couldn't match `[]' against `IO' Expected type: [t] Inferred type: IO Handle In the application `openFile (portsDir ++ (p ++ "/+CONTENTS")) ReadMode' In a 'do' expression pattern binding: handle <- openFile (portsDir ++ (p ++ "/+CONTENTS")) ReadMode i do not know what this [t] is and i do not know why it is expected. my theory is it is being caused by something in fetchDepends. fetchDepends :: Handle -> [String] fetchDepends handle = do l <- hGetLine handle e <- hIsEOF handle case (not e) of True -> case (matchRegex (mkRegex "^@pkgdep") l) of Just [a] -> [drop 8 l] ++ (fetchDepends handle) _ -> fetchDepends handle False -> [] here ghc reports Phoebe.hs:24: Couldn't match `[]' against `IO' Expected type: [t] Inferred type: IO String In the application `hGetLine handle' In a 'do' expression pattern binding: l <- hGetLine handle i am thinking that this has something to do with l. but i cant think of anything beyond that. here a couple of other attempts to show that i am trying and that i have no clue what i am doing. -- put a return around everything that might need it getDepends :: String -> [String] getDepends p = do handle <- openFile (portsDir ++ p ++ "/+CONTENTS") ReadMode return (fetchDepends handle) fetchDepends :: Handle -> [String] fetchDepends handle = do l <- hGetLine handle e <- hIsEOF handle case (not e) of True -> case (matchRegex (mkRegex "^@pkgdep") l) of Just _ -> return ([drop 8 l] ++ (fetchDepends handle)) _ -> return (fetchDepends handle) False -> return [] -- try making l into a string by show'ing it getDepends :: String -> [String] getDepends p = do handle <- openFile (portsDir ++ p ++ "/+CONTENTS") ReadMode fetchDepends handle fetchDepends :: Handle -> [String] fetchDepends handle = do l <- hGetLine handle e <- hIsEOF handle case (not e) of True -> case (matchRegex (mkRegex "^@pkgdep") (show l)) of Just _ -> [drop 8 (show l)] ++ (fetchDepends handle) _ -> fetchDepends handle False -> [] -- try removing the type declarations getDepends p = do handle <- openFile (portsDir ++ p ++ "/+CONTENTS") ReadMode fetchDepends handle fetchDepends handle = do l <- hGetLine handle e <- hIsEOF handle case (not e) of True -> case (matchRegex (mkRegex "^@pkgdep") l) of Just _ -> [drop 8 l] ++ (fetchDepends handle) _ -> fetchDepends handle False -> [] Phoebe.hs:27: Couldn't match `[[Char]]' against `IO [[Char]]' Expected type: [[Char]] Inferred type: IO [[Char]] In the application `fetchDepends handle' In the second argument of `(++)', namely `(fetchDepends handle)' Failed, modules loaded: none. ok. a different error message. it still makes no sense. i will try a couple more times before i finish this message. -- try show'ing (fetchDepends handle) getDepends p = do handle <- openFile (portsDir ++ p ++ "/+CONTENTS") ReadMode fetchDepends handle fetchDepends handle = do l <- hGetLine handle e <- hIsEOF handle case (not e) of True -> case (matchRegex (mkRegex "^@pkgdep") l) of Just _ -> [drop 8 l] ++ (show (fetchDepends handle)) _ -> fetchDepends handle False -> [] error message is Phoebe.hs:27: Couldn't match `IO' against `[]' Expected type: IO t Inferred type: [a] In the appli
Re: problems figuring out what the type system is telling me
see http://haskell.org/wiki/wiki?ThatAnnoyingIoType and http://haskell.org/wiki/wiki?UsingIo -- Hal Daume III "Computer science is no more about computers| [EMAIL PROTECTED] than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume On Fri, 7 Jun 2002, Chris Moline wrote: > hi. i am really stuck on this problem and i cant seem to figure it out and so > i am posting this in the hopes someone will help me. i realize you prolly get > this kind of question alot but the wiki and the papers i have been reading > arent helping to clarify things. > > i am trying to write a little utility to make it easier to get rid of ports on > my machine. the function that is giving me trouble is getDepends. it takes the > name of a port and opens the appropriate +CONTENTS file and returns the > dependencies it finds. > > here is the function and its helper. > > getDepends :: String -> [String] > getDepends p = do > handle <- openFile (portsDir ++ p) ReadMode > fetchDepends handle > > fetchDepends :: Handle -> [String] > fetchDepends handle = do > l <- hGetLine handle > e <- hIsEOF handle > case (not e) of -- ifs keep giving indent errors so ill just use case > True -> > case (matchRegex (mkRegex "^@pkgdep") l) of > Just [a] -> [drop 8 l] ++ (fetchDepends handle) > _ -> fetchDepends handle > False -> "" > > here is ghci's error messages. > > Compiling Pheobe ( Phoebe.hs, interpreted ) > > Phoebe.hs:19: > Couldn't match `[]' against `IO' > Expected type: [t] > Inferred type: IO Handle > In the application `openFile (portsDir ++ p) ReadMode' > In a 'do' expression pattern binding: > handle <- openFile (portsDir ++ p) ReadMode > > Phoebe.hs:24: > Couldn't match `[]' against `IO' > Expected type: [t] > Inferred type: IO String > In the application `hGetLine handle' > In a 'do' expression pattern binding: l <- hGetLine handle > Failed, modules loaded: none. > > could someone be so kind as to explain what the problem is? > > sincerly, > chris moline > ___ > Haskell-Cafe mailing list > [EMAIL PROTECTED] > http://www.haskell.org/mailman/listinfo/haskell-cafe > ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
problems figuring out what the type system is telling me
hi. i am really stuck on this problem and i cant seem to figure it out and so i am posting this in the hopes someone will help me. i realize you prolly get this kind of question alot but the wiki and the papers i have been reading arent helping to clarify things. i am trying to write a little utility to make it easier to get rid of ports on my machine. the function that is giving me trouble is getDepends. it takes the name of a port and opens the appropriate +CONTENTS file and returns the dependencies it finds. here is the function and its helper. getDepends :: String -> [String] getDepends p = do handle <- openFile (portsDir ++ p) ReadMode fetchDepends handle fetchDepends :: Handle -> [String] fetchDepends handle = do l <- hGetLine handle e <- hIsEOF handle case (not e) of -- ifs keep giving indent errors so ill just use case True -> case (matchRegex (mkRegex "^@pkgdep") l) of Just [a] -> [drop 8 l] ++ (fetchDepends handle) _ -> fetchDepends handle False -> "" here is ghci's error messages. Compiling Pheobe ( Phoebe.hs, interpreted ) Phoebe.hs:19: Couldn't match `[]' against `IO' Expected type: [t] Inferred type: IO Handle In the application `openFile (portsDir ++ p) ReadMode' In a 'do' expression pattern binding: handle <- openFile (portsDir ++ p) ReadMode Phoebe.hs:24: Couldn't match `[]' against `IO' Expected type: [t] Inferred type: IO String In the application `hGetLine handle' In a 'do' expression pattern binding: l <- hGetLine handle Failed, modules loaded: none. could someone be so kind as to explain what the problem is? sincerly, chris moline ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
