[Haskell-cafe] (Newbie Question) How to get the value of an "IO String" Monad?

[Daniel Fischer]

Hi,

Am Freitag, 17. Februar 2006 14:42 schrieb Peter:
> Hello,
>
> For the purpose of familiarizing myself with Haskell, and also because I
> love Haskell :),

very good!

> I am trying to re-make a script that I made in Python that sends a
> request to a server and extracts the list of email addresses from a
> Mailman web-page by using an XML Parser on the page's HTML that has been
> converted to XHTML by "HTML Tidy".
>
> However, I cannot seem to figure out a way to get the state of a Monad;
> Specifically I cannot get the value of an "IO String" Monad.
>
> I have read some tutorials on Monads but I guess I must have missed
> something.
>
> I have read that the >>= operator is the only way to extract the state
> of an action as a string, and pipe it to a function. So far so good.
> But, That does not seem to work, because as I understand the >>=
> operator, it expects the function on the right hand side to return an IO
> Monad, which completely defeats the purpose here.

I find do-notation often more readable, then you write val <- action and 
val is an ordinary value, you can use afterwards (within the same do-block, of 
course)

>
> So, How am I supposed to get the value of an IO Monad, such as "IO
> String", without returning an IO Monad?
>
> If this is of any help, here is the function I am stuck on:
> recv_headers' :: Socket.Socket -> String -> IO [[String]]
> recv_headers' sock bulk
>
>     | received == ""            = error "Connection died unexpectedly."
>     | received == "\n"
>
>       && endswith bulk "\r\n\r" = return [["foo", "bar"]]
>
>     | otherwise                 = recv_headers' sock (bulk ++ received)
>
>     where received = (Socket.recv sock 1)
> --- End code ---

This should do it:

recv_headers' :: Socket.Socket -> String -> IO [[String]]
recv_headers' sock bulk
     = do
     { received <- Socket.recv sock 1
     ; case received of
         "" -> error "Connection died unexpectedly."
         "\n" | endswith bulk "\r\n\r" -> return [["foo", "bar"]]
         _  -> recv_headers' sock (bulk ++ received)
     }

at least, it compiles.
>
> And here is the (expected) error I get from trying to compare "IO
> String" to "String":
> MemberBackup.hs:29:18:
>     Couldn't match `IO String' against `[Char]'
>       Expected type: IO String
>       Inferred type: [Char]
>     In the second argument of `(==)', namely `""'
>     In a pattern guard for
>        the definition of `recv_headers'':
>         received == ""
> Failed, modules loaded: none.
> --- End error ---
>
> Thanks for the help,
> Peter
>

Cheers,
Daniel
-- 

"In My Egotistical Opinion, most people's C programs should be
indented six feet downward and covered with dirt."
	-- Blair P. Houghton