problems figuring out what the type system is telling me

[Hal Daume III]

see http://haskell.org/wiki/wiki?ThatAnnoyingIoType and
http://haskell.org/wiki/wiki?UsingIo

--
Hal Daume III

 "Computer science is no more about computers    | hdaume@isi.edu
  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
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