Hi,
Apart from the other posts, you might also want to read
http://www.haskell.org/hawiki/IntroductionToIO which is a quick intro
to the way IO is handled in Haskell and
http://www.haskell.org/hawiki/UsingIo which covers similar ground, but
which also goes into a number of other common questions.
- Cale
On 17/02/06, Peter
Hello,
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 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.
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": 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
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