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