[Haskell-beginners] Problems when trying to solve yaht exercise 3.10

[Thomas Davie]

On 20 Mar 2009, at 13:45, ZelluX wrote:

> Hi, all
>
> I'm new to haskell and currently reading yaht. I find some problems  
> when trying to solve exercise 3.10.
>
> The exersices asks to read a list of numbers terminated by a zero,  
> and figure out the sum and product of the list. My program is as  
> follows:
>
> ex3_10 = do
>   hSetBuffering stdin LineBuffering
>   numbers <- getNumber
>   let sum = foldr (+) 0 numbers
>       product = foldr (*) 1 numbers
>   putStrLn "The sum is " ++ show(sum)
>   putStrLn "The product is " ++ show(product)
>
> getNumber = do
>   putStrLn "Give me a number (or 0 to stop):"
>   num <- getLine
>   if read num == 0
>      then return []
>      else do
>        rest <- getNumber
>        return (read num : rest)
>
> But when i load the program, ghci reports error:
>     Couldn't match expected type `[a]' against inferred type `IO ()'
>     In the first argument of `(++)', namely `putStrLn "The sum is "'
>     In a stmt of a 'do' expression:
>           putStrLn "The sum is " ++ show (sum)
>
> And i just don't understand the first sentence. Could you tell what  
> does it mean?

It means that you can't use ++ on an IO action, ++'s type reveals why:

(++) :: [a] -> [a] -> [a]

It accepts two lists, not a list and an IO action.  So the question  
then is, which IO action are you using ++ on?  The answer is (putStrLn  
"The sum is").  Your last two statements are being parsed as:

(putStrLn "The sum is ") ++ (show sum)
(putStrLn "The product is ") ++ (show product)

As a random aside, it's usually a good plan in Haskell to get out of  
IO based computations as fast as possible, and use pure functions  
instead.  You might want to consider something like this:

ex3_10 = do
   hSetLineBuffering stdin LineBuffering
   interact sumAndProduct

sumAndProduct :: String -> String
sumAndProduct = (\ns -> sumText ns ++ productText ns) . map read . lines

sumText :: (Num a, Show a) => [a] -> String
sumText = ("The sum is " ++) . show . sum

productText :: (Num a, Show a) => [a] -> String
productText = ("The product is " ++) . show . product

Why might you want to do this?  Well, firstly, the code becomes more  
readable, secondly, a computation that was not sequential is no longer  
described as being sequential, and finally, the results are more  
composible.  We can now use sumText and productText, safe in the  
knowledge that they will never have side effects etc.

Bob