[Haskell-cafe] REALLY simple STRef examples

[Shao Chih Kuo]

Yes, largely the choice to define foreach was made to try and make it 
look more imperative, I showed it to an imperative programmer to try and 
convince him that you could program in an imperative way in Haskell if 
you really wanted to, that and I thought it'd an imperative style would 
make an interesting addition to the evolution of a Haskell programmer.

Bulat Ziganshin wrote:
> Hello Bryan,
>
> Saturday, July 22, 2006, 4:40:58 AM, you wrote:
>   
>> Forgive me for not understanding, but I was hoping you would explain a
>> choice you made in your code. Why did you define foreach and then use
>>     
>
>   
>>> foreach [1..n] (\x -> modifySTRef r (*x))
>>>       
>
>   
>> Instead of simply using
>>     
>
>   
>>> mapM_ (\x -> modifySTRef r (*x)) [1..n]
>>>       
>
> because it looks just like for/foreach loops in imperative languages.
> look at this:
>
> import Control.Monad
> import Data.IORef
>
> infixl 0 =:, +=, -=, .=, <<=
> ref = newIORef
> val = readIORef
> a=:b = writeIORef a b
> a+=b = modifyIORef a (\a-> a+b)
> a-=b = modifyIORef a (\a-> a-b)
> a.=b = ((a=:).b) =<< val a
> for :: [a] -> (a -> IO b) -> IO ()
> for = flip mapM_
>
> newList = ref []
> list <<= x   =  list =:: (++[x])
> push list x  =  list =:: (x:)
> pop list     =  do x:xs<-val list; list=:xs; return x
>
> main = do
>   sum <- ref 0
>   lasti <- ref undefined
>   for [1..5] $ \i -> do
>     sum += i
>     lasti =: i
>   sum .= (\sum-> 2*sum+1)
>   print =<< val sum
>   print =<< val lasti
>
>   xs <- newList
>   for [1..3] (push xs)
>   xs <<= 10
>   xs <<= 20
>   print =<< val xs
>
>
>
>