problems figuring out what the type system is telling me

[Chris Moline]

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 application `(++) [drop 8 l] (show (fetchDepends handle))'
    In a case alternative: [drop 8 l] ++ (show (fetchDepends handle))

great another error message that is meaningless to me. one last attempt.

-- try adding the type decs back into the above code
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") l) of
                                Just _ -> [drop 8 l] ++
                                        (show (fetchDepends handle))
                                _ -> fetchDepends handle
                False -> []

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

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  back to the old message none the wiser. please help me. i am going 
insane.

sincerly,
chris moline