[Haskell-cafe] Properties of monads

[Benja Fallenstein]

Hi Radosław,

You should be able to write this with MaybeT as follows:

getStrip :: IO ( Maybe String )
getStrip = runMaybeT $ do
    pageContent <- liftIO $ downloadFile mainPageAddress
    let x = patternForStrip pageContent
    print x
    z <- x
    liftIO $ downloadFile $ mainPageAddress ++ z

If you can do without the 'print', you should be able to write it as:

getStrip :: IO ( Maybe String )
getStrip = runMaybeT $ do
    pageContent <- liftIO $ downloadFile mainPageAddress
    z <- patternForStrip pageContent
    liftIO $ downloadFile $ mainPageAddress ++ z

You can find MaybeT here:

http://www.haskell.org/haskellwiki/New_monads/MaybeT

Best,
- Benja

On 11/18/07, Radosław Grzanka <radoslawg at gmail.com> wrote:
> Hello,
>   I am writing some toys programs to learn and try to apply Monads
> properties (without success, I must say). Although I spent half a day
> on this code:
>
> http://hpaste.org/3957
>
> I couldn't simplify (shorten) getStrip function. After reading "Doing
> it with class" (
> http://www.haskell.org/all_about_monads/html/class.html ) I had an
> impression that I could "collapse" cases using some Monads properties.
> But maybe I misunderstood something.
>
> Can anyone look at it and give me a pointers?? (I don't mind if the
> code becomes unreadable a bit.)
>
> Thank you,
>   Radek.
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