On Sat, Jun 24, 2006 at 02:38:31PM +1000, Geoffrey King wrote:
I have been work my way through "Haskell The Craft of Functional Programming", all was fine until IO (chapter 18). That is causing me bafflement.
I am trying to write a totally trivial program that reads a series of integers from the console until it gets a zero, then returns the series of integers as a list.
I am down to randomly editing my code, never a good sign. Any clues would be appreciated, My code so far, which clear doesn't work, is: getInt :: IO Int getInt = do putStr "Enter number (zero to quit)" line <- getLine return (read line :: Int)
anIntList :: [Int]
anIntList is an IO action which reads a list, and as such has type IO [Int], not [Int].
anIntList = do let n = getInt
You should use "n <- getInt" to perform getInt and read integer into n. "let n = getInt" just says that n is an IO action which reads integer, not that integer itself.
if n == 0 then return [] else return (n : anIntList)
As anIntList is not a list this won't work. You should perform anIntList and then use its result: do list <- anIntList return (n : list)