Hi, Rather than using indexing, or using head, I recommend using pattern matching. These functions are not total, i.e. they fail to provide a valid result for some specific inputs, like head on empty list. The following is similar to replicateM from Control.Monad, you can either modify it to suit your need, or use replicateM combined with map to turn a list of Strings to a list of IO () as required by replicateM. execNTimes :: Int -> [IO ()] -> IO () execNTimes 0 [] = return () execNTimes n (x : xs) | n <= 0 = return () | otherwise = x >> execNTimes (n - 1) xs Where, the following are equivalent: a >> b == do a b Regards, Sumit On 9 May 2016 at 19:00, Henson wrote:

Hi,

I'm new in Haskell and I need help in recursion. That function below is returning "*** Exception: Prelude.head: empty list" and I need resolve that: execNTimes 0 [] = return() execNTimes n xs = if n<=0 || null xs then return() else do si <- getLine let s = words si l = read (s !! 1) :: Int r = read (s !! 2) :: Int if head s=="Q" then do let z = slice l r xs m = foldl lcm 1 z print (m `mod` (toInteger 1000000007)) else do let s1 = update l r xs execNTimes (n-1) s1 execNTimes (n-1) xs

Anybody can me help?

Thank you, Josenildo Silva _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

Hi Josenildo. I would personally rewrite your code to something like this: import Data.Maybe (fromMaybe) import System.IO (hFlush, stdout) import Text.Read (readMaybe) readDef :: a -> String -> a readDef def = fromMaybe def . readMaybe isAtLeast :: [a] -> Int -> Bool isAtLeast ls n | n <= 0 = True | null ls = False | otherwise = isAtLeast ls (n-1) execNTimes :: Int -> [a] -> IO () execNTimes n lst | n <= 0 = return() | do (from, to, quit) <- getCmds if quit then let z = slice from to xs m = foldl lcm 1 z in print $ m `mod` modVal else execNTimes (n-1) $ update from to lst execNTimes (n-1) lst where modVal :: Integer modVal = 1000000007 getCmds :: IO (Int, Int, Bool) getCmds = do putStr "Enter command: " hFlush stdout cmds <- words <$> getLine if not $ cmds `isAtLeast` 3 then do putStrLn "Too few commands" getCmds else let from = readDef 0 (cmds !! 1) :: Int to = readDef 0 (cmds !! 2) :: Int quit = head cmds == "Q" in if from < 0 || to >= length lst || from > to then do putStrLn "Invalid range" getCmds else (from, to, quit) Do note that the above code is untested, and that it's written on a mobile phone. I don't know what your intentions with the code are, so I added some error messages and variable names that you might want to change. About 'xs': xs is frequently used to mean "rest of list". I would recommend using ls or lst instead to name the whole list.

On Mon, May 9, 2016 at 6:30 AM, Henson wrote:

execNTimes 0 [] = return() execNTimes n xs = if n<=0 || null xs then return() else do si <- getLine let s = words si l = read (s !! 1) :: Int r = read (s !! 2) :: Int if head s=="Q" then do let z = slice l r xs m = foldl lcm 1 z print (m `mod` (toInteger 1000000007)) else do let s1 = update l r xs execNTimes (n-1) s1 execNTimes (n-1) xs

Your code is making the assumption that (words si) is non-empty. When that's not true, such as when the input from the console is empty, your program will crash at the use of `head`, since it throws an error when applied to an empty list. That's what you are seeing. Incidentally, by using `l` and `r` later on, you're also assuming that the input contains at least 2 more words after that first one. Violating that will also crash your program. Finally, your use of `read` means that the program will crash if the second two inputs are not numbers. All three of these functions should be used with great caution, or not at all, since they threaten to crash your program! Here's a very concise way to write this, using pattern guards: case words si of [ a, ls, rs ] , Right l <- readEither ls :: Either String Int , Right r <- readEither rs :: Either String Int -> if a == "Q" then do let x = slice l r xs let m = foldl lcm 1 z print (m `mod` (toInteger 1000000007)) else do let s1 = update l r xs execNTimes (n-1) s1 _ -> putStrLn "Invalid input."