foreach Re[2]: [Haskell-cafe] REALLY simple STRef examples

[Bulat Ziganshin]

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



-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com