Re: [Haskell-cafe] foreach

On Wed, 13 Sep 2006, Donald Bruce Stewart wrote:

lemmih:

...

On 9/13/06, Tim Newsham wrote:

...

I was rewriting some non-haskell code in haskell and came up with this construct:

foreach l f = mapM_ f l

main = do args <- getArgs foreach args (\arg -> do foreach [1..3] (\n -> do putStrLn ((show n) ++ ") " ++ arg) ) )

which is reminiscent of foreach in other languages. Seems fairly useful and I was wondering how hard it would be to add some syntactic sugar to the "do" construct to make it a little prettier (ie. not require the parenthesis, binding and nested do, as:

main = do args <- getArgs foreach args arg foreach [1..3] n putStrLn ((show n) ++ ") " ++ arg)

would this type of transformation be possible with template haskell or does this need stronger support from the parser to pull off?

How about:

main = do args <- getArgs flip mapM_ args $ \arg -> flip mapM_ [1..3] $ \n -> putStrLn $ show n ++ ") " ++ arg

Which is, with current Control.Monad:

main = do args <- getArgs forM_ args $ \arg -> forM_ [1..3] $ \n -> putStrLn $ show n ++ ") " ++ arg

I think Tim is looking for an if-then-else "real syntax" feel to his `foreach' though. I.e. TH or some small preprocessor.

Adding sugar or using Template Haskell for such a simple task is a bit unreasonable. I think Tim should use mapM a little bit and then he will probably need no longer a special syntax. If you want more sugar, what about the list monad? main = do args <- getArgs sequence_ $ do arg <- args n <- [1..3] return (putStrLn $ show n ++ ") " ++ arg) or main = do args <- getArgs sequence_ $ liftM2 (\arg n -> putStrLn $ show n ++ ") " ++ arg) args [1..3]