Daniel Carrera wrote:
Hello,
I've been studying more Haskell and I've improved a lot. But I just hit a small problem. I want to print all the elements of a linst (putStr). I'd like to write something like this:
print_list [] = do putStr "" print_list (x:xs) = (do putStr x) && print_list xs
I know this is wrong, but I hope you can see what I'm trying to do.
I know of other ways I could print a list. For example:
print_list xs = do putStr(join xs) where join [] = "" join (x:xs) = (show x) ++ "\n" ++ join xs
But the thing is, I want to write a lazy version of this function. It's not that I need such a function, I'm just trying to learn Haskell.
Any suggestions?
Question: What do you call a function that has side-effects? (like putStr) I know that "function" is the wrong term.
Cheers, Daniel.
I sometimes call a function with side-effects in IO a "command". But the terms are fungible. But calling putStr a "function" is correct. It is not a "pure function" however. What does lazy printing mean? I assume it means you evaluate the head of the list, print it, then recursively do this for the tail of the list. With an infinite list you will get inifinite output. I assume it does not mean you evaluate the whole list before printing anything. This would prevent infinite lists from producing output. (mapM_ putStr) or (mapM_ putStrLn) will do what you want. All of these commands show work, even if hw is inifitely long: let printList [] = return () printList (x:xs) = do putStrLn x printList xs main = do let hw = ["Hello"," ","World","!"] mapM_ putStr hw mapM_ putStrLn hw putStr (unlines hw) -- see also: unwords, words, lines printList hw -- Chris