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

[Daniel Fischer]

Am Montag, 15. Dezember 2008 23:26 schrieb Jeff C. Britton:
> 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

There's another one lurking there, 
:t sqrt
sqrt :: (Floating a) => a -> a

but Integer is not a member of class Floating. The one displayed says that you 
must have an IO-action in your do-block, not a plain list of numbers.

do nums <- askForNumbers
   printList $ map (sqrt . fromInteger) nums

works, or

printList . map (sqrt . fromInteger) =<< askForNumbers

>
> 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
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