[Haskell-cafe] Channel9 Interview: Software Composability and the Future of Languages

[Tomasz Zielonka]

On Wed, Jan 31, 2007 at 07:46:15PM +0300, Bulat Ziganshin wrote:
> Wednesday, January 31, 2007, 12:01:16 PM, you wrote:
> 
> >> there are also many other similar issues, such as lack of good syntax
> >> for "for", "while", "break" and other well-known statements,
> 
> > On the other hand you have an ability to define your own control
> > structures.
> 
> i have a lot, but their features are limited, both in terms of
> automatic lifting and overall syntax. let's consider
> 
> while (hGetBuf h buf bufsize == bufsize)
>   crc := updateCrc crc buf bufsize
>   break if crc==0
>   print crc

A direct translation could look like this:

    whileM c b = do { x <- c; when x (b >> whileM c b) }

    f h buf =
        flip runContT return $ do
            callCC $ \break -> do
                flip execStateT 0 $ do
                    whileM (liftIO (liftM (== bufsize) (hGetBuf h buf bufsize))) $ do
                        do  crc <- get
                            crc' <- liftIO (updateCrc crc buf bufsize)
                            put crc'
                        crc <- get
                        when (crc == 0) (lift (break crc))
                        liftIO (print crc)

Which, admittedly, is much more lengthy. If we assume that hGetBuf,
updateCrc and print can work in any MonadIO, and we define
    inContT x = flip runContT return x
then it becomes slightly shorter:

    inContT $ callCC $ \break -> do
        flip execStateT 0 $ do
            whileM (liftM (== bufsize) (hGetBuf h buf bufsize)) $ do
                do  crc <- get
                    crc' <- updateCrc crc buf bufsize
                    put crc'
                crc <- get
                when (crc == 0) (lift (break crc))

Let's define:

    modifyM f = do
        x <- get
        x' <- f x
        put x'

and change the order of parametrs in updateCrc. We get:

    inContT $ callCC $ \break -> do
        flip execStateT 0 $ do
            whileM (liftM (== bufsize) (hGetBuf h buf bufsize)) $ do
                modifyM (updateCrc buf bufsize)
                crc <- get
                when (crc == 0) (lift (break crc))
                print crc

> how this can be expressed in Haskell, without losing clarity?
 
I think it's quite clear what it does.

> > "inability" is an exaggeration - you can use the ContT monad
> > transformer, which even allows you to choose how "high" you
> > want to jump. But you probably already know this and just want to point
> > that it is cumbersome?
> 
> don't know and don't want to use such a hard way.

I gave an example above. You can "break" with a return value, so it
seem it's what you want.

> there is a simpler solution, but it still requires to write more
> boilerplate code than C:
> 
> res <- doSomething
> if isLeft res  then return$ fromLeft res  else do
> let (Right x) = res
> ...

Not simpler, but easier... and uglier. Somehow I don't like to solve
problems on the level of programming language syntax.

Best regards
Tomasz