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

Peter peter at commonlawgov.org
Fri Feb 17 08:42:09 EST 2006


For the purpose of familiarizing myself with Haskell, and also because I
love Haskell :),
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

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.

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 ---

And here is the (expected) error I get from trying to compare "IO
String" to "String":
    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,

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