[Haskell-cafe] Newbied question on IO Monad

[Sara Kenedy]

Hello,

Maybe what I talk is not clear.

I want to take the input string sss of "update" to use in: writeFile
"myFile.txt" sss of function "main" and get the value zzz from "main"
to assign for the value return of "update". I think I need to change
the way to declare two functions to get what I want, but I do not know
how.

update :: String -> String
update sss = zzz

main = do writeFile "myFile.txt" sss
                x <- callSystem "myFile.txt"
                y <- openFile "result.txt" ReadMode
                zzz <- hGetContents y
                return zzz

S.

On 9/12/06, Andrea Rossato <mailing_list at istitutocolli.org> wrote:
> Il Tue, Sep 12, 2006 at 08:05:42AM -0400, Sara Kenedy ebbe a scrivere:
> > Hello all,
> >
> > update :: String -> String
> > update sss = ...
> >
> >
> > main = do writeFile "myFile.txt" sss
> >                x <- callSystem "myFile.txt"
> >                y <- openFile "result.txt" ReadMode
> >                zzz <- hGetContents y
> >                return zzz
> >
> >
> > I know that function main returns IO String, function update returns
> > String. And my purpose is to get the string zzz from main for the
> > value return of function update. But I do not know which way I can do.
>
> Did you mean something like this?
>
> update :: String -> String
> update sss = "Hi! " ++ sss
>
> main = do writeFile "myFile.txt" $ update "What are you trying to do?"
>           x <- callSystem "myFile.txt"
>           y <- openFile "result.txt" ReadMode
>           zzz <- hGetContents y
>           return zzz
>
> In this case main :: IO String so you will not see any output (quite
> useless).
> I'd suggest you to have a look at this tutorial that explain quite
> well the IO Monad:
> http://haskell.org/haskellwiki/IO_inside
>
> Ciao
> Andrea
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