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As you describe the problem, there is no need for state at all: <pre><code>data Browser = Browser { getUrl :: String, getHtml :: String, getLinks :: [String]} getLinksFromHtml :: String -> [String] -- use Text.HTML.TagSoup, it should be lazy goto :: String -> IO Browser goto url = do -- assume getUrlContents is lazy, like hGetContents html <- getUrlContents url let links = getLinksFromHtml html return (Browser url html links) </code></pre> <blockquote> It’s possbile to have 2 or “browsers” at once, with its own separate state: </blockquote> You obviously can have as many as you want, and they can't interfere with each other. Now the equivalent of your snippets. First: <pre><code>htmlFromGooglesFirstLink = do b <- goto "http://www.google.com" let firstLink = head (links b) b2 <- goto firstLink -- note that a new browser is returned putStr (getHtml b2) </code></pre> And second: <pre><code>twoBrowsers = do b1 <- goto "http://www.google.com" b2 <- goto "http://www.stackoverflow.com/" putStr (getHtml b1) putStr (getHtml b2) </code></pre> UPDATE (reply to your update): <blockquote> If Browser has a state, it can send HTTP Referer header and cookies while hiding all mechanics inside itself and giving nice API. </blockquote> No need for state still, <code>goto</code> can just take a Browser argument. First, we'll need to extend the type: <pre><code>data Browser = Browser { getUrl :: String, getHtml :: String, getLinks :: [String], getCookies :: Map String String } -- keys are URLs, values are cookie strings getUrlContents :: String -> String -> String -> IO String getUrlContents url referrer cookies = ... goto :: String -> Browser -> IO Browser goto url browser = let referrer = getUrl browser cookies = getCookies browser ! url in do html <- getUrlContents url referrer cookies let links = getLinksFromHtml html return (Browser url html links) newBrowser :: Browser newBrowser = Browser "" "" [] empty </code></pre> <blockquote> If Browser has no state, the developer is likely to pass around all current URL/HTML/Cookies -- and this adds noise to scenario code. </blockquote> No, you just pass values of type Browser around. For your example, <pre><code>useGoogle :: IO () useGoogle = do b <- goto "http://www.google.com/" newBrowser let b2 = typeIntoInput 0 "haskell" b b3 <- clickButton "Google Search" b2 ... </code></pre> Or you can get rid of those variables: <pre><code>(>>~) = flip mapM -- use for binding pure functions useGoogle = goto "http://www.google.com/" newBrowser >>~ typeIntoInput 0 "haskell" >>= clickButton "Google Search" >>= clickLink "2" >>= clickLink "3" >>~ getHtml >>= putStr </code></pre> Does this look good enough? Note that Browser is still immutable.
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