Re: [Haskell-cafe] using the writer monad to better understand foldl and foldr, and haskell debugging techniques in general

On Feb 10, 2008 9:33 PM, Thomas Hartman wrote:

-- using writer monad -- Nothing unsafe here, pure referrentially transparent goodness myfoldrW f z [] = return z myfoldrW f z (x:xs) = do r <- (myfoldrW f z xs) tell ("x,r: " ++ (show (x,r)) ++ "\n" ) return $ x `f` r

*Main> foldr const 0 [1..] 1 *Main> putStrLn $ snd $ runWriter $ myfoldrW const 0 [1..] Interrupted. One of the good things from foldr is the possibility of "short-circuiting", so to speak. However I don't know if it is possible to show this using the writer monad, as is would involve observing if the function is strict or not in its second argument. Cheers, -- Felipe.