Re: [Haskell-cafe] I'd like start with Haskell, but...

Bulat Ziganshin wrote:

Hello Waldemar,

Sunday, December 17, 2006, 2:44:28 AM, you wrote:

...

Maybe, but what is still unclear for me: Haskell is wrong for GUI/Database application because of lack of good libraries or because of it's way of programming???

primarily, first. to some degree, second too. Haskell doesn't provide such elegant representation of imperative algorithms as imperative languages does. just for example, counting sum [1..n]:

I see that "n" is not in an IORef, and for n=1 your loop leaves sum holding 0, which is just the kind of off-by-1 fencepost but that c excels in generating:

sum <- newIORef 0 i <- newIORef 1 let go = do i' <- readIORef i when (i'

compare this to C code :) of course, you can define a lot of sugar/combinators that simplify such code but anyway C will be better in this area

c code: int sum(int n) { int i = 1; int result = 0 for (; i<=n ; ++i) { result += i; } return result; } haskell: You really want this to be strict, so replace modifyIORef with modifyIORef'

modifyIORef' r f = do new <- liftM f (readIORef r) writeIORef r $! new return new

s1 n = do sum <- newIORef 0 forM_ [1..n] $ \i -> modifyIORef' sum (+i) readIORef sum

As you mentioned, there is easy syntactic sugar definable in Haskell itself:

a .+=. b = do new <- liftM2 (+) (readIORef a) (readIORef b) writeIORef a $! new

a .<=. b = liftM2 (<=) (readIORef a) (readIORef b)

succ_ i = modifyIORef' i succ

for init test inc block = do init let loop = do proceed <- test when proceed (block >> inc >> loop) loop

With that sugar it becomes easy to look very much like the c-code:

s2 n = do sum <- newIORef 0 i <- newIORef 1 for (return ()) (i .<=. n) (succ_ i) (sum .+=. i) readIORef sum

And we do have "postfix" operators with the latest GHC:

(.++) a = do old <- readIORef a writeIORef a $! succ old return old