[Haskell-cafe] loop in C , recursion in haskell

[Duncan Coutts]

On Wed, 2004-03-24 at 15:51, Simon Marlow wrote:
> > > I am new to haskell and would look to write a function equivalent
> > > to the following loop in C
> > > 
> > > int value = 500000;
> > > int part_stack[4];
> > > int *part_ptr = part_stack;
> > > for (; value; value /= 10000)
> > >       *part_ptr++ = value % 10000;
> > 
> > part_stack :: [Int]
> > part_stack = [0,50]
> > 
> > Note that I've performed a memoization optimization--this 
> > makes the code both smaller, faster and easier to read! :P
> 
> Ignore David, he's pulling your leg.  Here's the proper translation:
> 
>  do
>   alloca $ \value -> do
>   poke value (500000::Int)
>   allocaArray 4 $ \part_stack -> do
>   alloca $ \part_ptr -> do
>   poke part_ptr part_stack
>   let loop = do
>            val <- peek value
>            if val == 0 then return () else do
>            p <- peek part_ptr
>            poke p (val `rem` 10000)
>            poke part_ptr (p `plusPtr` 1)
>            poke value (val `quot` 10000)
>            loop
>   loop

Yes, very nice.

Poor Simon, he's clearly been writing too much low-level FFI code
recently. :-)

Duncan