Well, if you want that in production, not for debugging purposes, you should change the type signature of mergesort so that it uses some monad. Printing requires IO monad; however, I would advise to collect all intermediate results using Writer monad, and print them afterwards:
mergesort [] = return [] mergesort [x] = return [x] mergesort xs = do tell [xs] -- that's right, "[xs]", not "xs" let (as, bs) = splitAt (length xs `quot` 2) xs liftM2 merge (mergesort as) (mergesort bs) On 4 Feb 2012, at 22:23, Qi Qi wrote: > Hello, > > I have a question;how can I print out the intermediate number lists in a > mergesort recursive function like the following one. > > merge [] ys = ys > merge xs [] = xs > merge (x:xs) (y:ys) = if x <= y > then x : merge xs (y:ys) > else y : merge (x:xs) ys > > mergesort [] = [] > mergesort [x] = [x] > mergesort xs = let (as, bs) = splitAt (length xs `quot` 2) xs > in merge (mergesort as) (mergesort bs) > > main = do > print $ mergesort [5,4,3,2,1] > > > In the main function, it only prints out the final number list. But I'd > like to print out the number lists in every recursive level. How can I > do that? Thanks. > > > -- > Qi Qi > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
