On Mon, Dec 15, 2008 at 12:17 PM, Jeff C. Britton
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.pdfhttp://www.cs.utah.edu/%7Ehal/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