[Haskell-beginners] Problems with one of my first examples

Jeff C. Britton jcb at iteris.com
Mon Dec 15 15:17:33 EST 2008


I have started reading "Yet Another Haskell Tutorial" by Hal Daum´e III which can be found here 

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]]

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

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?

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]


Is there a way to write printList to handle Strings or numbers?
Or should I write
printList (map show askForNumbers)



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