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

same behavior with myfoldrD (:) [] [1..] -- uses Debug.Trace.trace So, I would say this proves my main point, which was that you could accomplish the same thing using the writer monad that you could do using the more "ad hoc" trace function from Debug.Trace. It's good that you point this out though, because understanding that foldr can take an infinite list and foldl not is a very key point.

2008/2/10, Felipe Lessa :

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

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-- 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.