Thanks, Michael. Ok, I understand most of what you did, but ... do nums <- askForNumbers; map sqrt nums ERROR - Type error in final generator *** Term : map sqrt nums *** Type : [Integer] *** Does not match : IO a do nums <- askForNumbers; printList nums -- works fine Thanks, Jeff ------------------------------------------------------------- Hello, I have started reading "Yet Another Haskell Tutorial" by Hal Daum´e III which can be found here http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf One of the early examples in section 3.8 pg. 35 is this askForWords = do  putStrLn "Please enter a word:"  word <- getLine  if word == ""    then return []    else do      rest <- askForWords      return (word : rest) I want to print the returned list and everything I try fails. I have tried the following: printList l =  if length l >= 1    then do putStrLn (head l)            printList (tail l)    else putStrLn("") f = printList askForWords and I get Expression     : printList askForWords *** Term           : askForWords *** Type           : IO [[Char]] *** Does not match : [[Char]] I believe one of the following will work for you: f = askForWords >>= printList f = do         words <- askForWords         printList words   ************************************* The exercise right below this asks for a very slight modification to read numbers instead. However, I am confused about how to convert strings to numbers. If I type in the hugs interactive console read "5" + 3 --> 8     -- ok perfect However read "5" gives ERROR - Unresolved overloading *** Type       : Read a => a *** Expression : read "5" Yet page 33 of the tutorial has the following code: doGuessing num = do  putStrLn "Enter your guess:"  guess <- getLine  let guessNum = read guess  -- ok in let stmt, but not at repl prompt? The problem here is type inference. The statement read "5" has type "(Read a) => a", which basically means anything that implements the class "Read." When you do read "5" + 3, the read "5" gets the type of the 3. I assume that in the latter case, you use the expression guessNum in a way later on that the compiler can infer its type. Anyway I take the info that has been presented and create this function: askForNumbers = do    hSetBuffering stdin LineBuffering    putStrLn "Give me a number (or 0 to stop)"    numStr <- getLine    let num = read numStr    if num == 0        then return []        else do            rest <- askForNumbers            return (num : rest) However, when I try to use it, like say map sqrt askForNumbers ERROR - Type error in application *** Expression     : map sqrt askForNumbers *** Term           : askForNumbers *** Type           : IO [Integer] *** Does not match : [a] Similar to above, try this: do nums <- askForNumbers     map sqrt nums   ********************************************************* Is there a way to write printList to handle Strings or numbers? Or should I write printList (map show askForNumbers) Note: you should probably do this using mapM_, but for simplicity, I'll do it using explicit recursion: printList [] = putStrLn "" -- or return () if you don't want the extra blank line printList (x:xs) = do putStrLn (show x)                       printList xs If you have any questions about how these worked, let me know! Michael

Jeff C. Britton wrote:

do nums <- askForNumbers; map sqrt nums

ERROR - Type error in final generator *** Term : map sqrt nums *** Type : [Integer] *** Does not match : IO a

Here (map sqrt nums) has type [Integer], so you cannot use it in a do expression for IO. Instead, you have to write something which produces an IO action.

do nums <- askForNumbers; printList nums -- works fine

Here, (printList nums) has type IO (), so it fits into the do expression and everything is fine. If you want to print the list of square roots, you have to say so: do nums <- askForNumbers; printList (map sqrt nums) Tillmann