On 7/21/06, S C Kuo wrote:

Not totally relevant to what the discussion has evolved to, but I wrote a factorial function using STRefs (in the spirit of the Evolution of a Haskell programmer) and I think it qualifies as a really simple example. Code follows:

import Data.STRef import Control.Monad.ST

foreach :: (Monad m) => [a] -> (a -> m b) -> m () foreach = flip mapM_ -- Bryn Keller's foreach, but with type restrictions

fac :: (Num a, Enum a) => a -> a fac n = runST (fac' n)

fac' :: (Num a, Enum a) => a -> ST s a fac' n = do r <- newSTRef 1 foreach [1..n] (\x -> modifySTRef r (*x)) x <- readSTRef r return x

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]

? I tried it out in GHCi, and it worked fine, and I have seen code that has been defined as a flip to take advantage of partial application. But your code doesn't seem to take advantage of partial application, so why did you define 'foreach' and then use it instead of using 'mapM_'? I am just curious, and have always been interested in reasons behind coding style. Bryan Burgers

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