Monadify evented code

Monadify evented code OR: how to put synchronicity back in asynchronous code.
Here i describe how to use CoffeeScript in order to port the "synchronous" programming style to the evented, turn based Model used by node.js

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1. Monadify evented code Or how to put synchronicity back in asynchronous code
2. In node.js code looks sometimes like this readRequest(conn,function(req){ readTemplateFile(fn,function(data){ parseTemplateFile(data,function(tepl){ queryDataBase(„…..",function(resultset){ fetchNextResult(resultset,function(row){ sendResponse(conn,...); }); }); }); }); });
3. Now lets look at Haskell (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b Nothing >>= _ = Nothing (Just x) >>= f = f x return :: a -> Maybe a return x = Just x add :: Maybe Int -> Maybe Int -> Maybe Int add mx my = mx >>= (\x -> my >>= (\y -> return (x + y)))
4. Now, Haskell with syntactic sugar (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b Nothing >>= _ = Nothing (Just x) >>= f = f x return :: a -> Maybe a return x = Just x add :: Maybe Int -> Maybe Int -> Maybe Int add mx my = do x <- mx y <- my return (x + y)
5. Lessons, that we lerned • Monads are some kind of masked values – type<a> M<a> • The >>= operator extracts a from M<a> – operator<a,b> M<a> >>= (a -> M<b>) -> M<b> • The >>= operator may delay the execution of the function • And that‘s it.
6. Applying what we lerned back to node.js • First we need the foundation for this • Javascript has – No operator overloading – No >>= operator – Is dynamically typed • Our requirements are: – Something thats to use like a monad – Some replacement for the >>= operator
7. Defining JS equivalents Haskell 1. a >>= func 2. Monad value 3. y <- f(x) Javascript 1. a(func) 2. function( func ){…} 3. f(x)(function(y){…})
8. So we need to redefine our api a bit Whereever we get: apiFunction(param1,param2,callback) Well redefine it as: apiFunction(param1,param2)(callback)
9. Then the code we get at first looks like this readRequest(conn)(function(req){ readTemplateFile(fn)(function(data){ parseTemplateFile(data)(function(tepl){ queryDataBase("S…")(function(resultset){ fetchNextResult(resultset)(function(row){ sendResponse(conn,...); }); }); }); }); });
10. Wait a second! Doesn‘t this also look clunky in almost the same way?
11. Yes! Thats why well embrace CoffeeScript
12. In CoffeeScript our code would look like this readRequest(conn) (req)-> readTemplateFile(fn) (data)-> parseTemplateFile(data) (tepl)-> queryDataBase("S...") (resultset)-> fetchNextResult(resultset) (row)-> sendResponse(conn,...)
13. And with even more syntactic sugar readRequest(conn) (req)->\ readTemplateFile(fn) (data)->\ parseTemplateFile(data) (tepl)->\ queryDataBase("S...") (resultset)->\ fetchNextResult(resultset) (row)->\ sendResponse(conn,...)
14. Hey, whats about loops?
15. Thats about loops ;-) repeat((recur,end,whil)-> s.readLine(10) (data)->\ print('XML> '+data) ->\ w.write(data) ->\ whil(data.length) ) ->\ more… # and you can also use # tail recursion