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

1 Feb 2007

      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

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

Tomasz Zielonka