Haskell IO function accepting return value from a another function

Question

module PRO where
average1 :: IO Float
avarage1  =
    do
        putStrLn "Enter Marks in Form of a List"
        marks <- getLine
        let m = (read marks)::[Int]
        x<-(sum' (m)) 
        avg <- x/length (m)
        if(x/=[])
           then
                putStrLn ("empty List")
            else
                putStrLn ("Your Avarage is " ++ show(avg))


sum' :: (Num a) => [a] -> a
sum' xs = foldl (\acc x -> acc + x) 0 xs

My program doesn't seems to work! My question is why can't I assign avg the returning sum of the sum' function?

vivian · Accepted Answer · 2011-06-12 04:40:02Z

6

module PRO where

average1 :: IO ()
average1 =
    do
        putStrLn "Enter Marks in Form of a List"
        marks <- getLine
        let m = (read marks)::[Int]
        let x = sum' m 
        let avg = (fromIntegral x)/(fromIntegral $ length m)
        if(m==[])
            then
                putStrLn ("empty List")
            else
                putStrLn ("Your Avarage is " ++ (show avg))

sum' :: (Num a) => [a] -> a
sum' xs = foldl (\acc x -> acc + x) 0 xs

i) The type signature of average1 is incorrect, the function does not return a value

ii) [Edit: this point was incorrect]

iii) the average value is a float, so you need to cast the integer arguments

iv) Your test if (x/=[]) is the wrong way round and should use m not x

v) most of your lines are not in the IO monad and so should use let inside a do block

edited Jun 12, 2011 at 4:40

answered Jun 12, 2011 at 3:14

vivian

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2 Comments

Visuddha Karunaratne Over a year ago

MacPhail thanks a lot for your help. The code compiled no problem but i still get a run time error saying PRO> average1 Enter Marks in Form of a List [1,2,3] Your Avarage is Program error: Prelude.read: no parse

Visuddha Karunaratne Over a year ago

@jwodder @Vivian McPhail got it, thanks for the help you guys provided. you rock!

jwodder · Accepted Answer · 2011-06-12 03:04:52Z

5

You can't use the <- notation for assigning the return values of sum' m and x/length m. <- can only be used when the right-hand side is a monadic expression (in this case, an IO value), which neither one is, and so you should use let x = sum' m and let avg = x / fromInteger (length m) instead (fromInteger is needed to convert the Int returned by length m to a Fractional value so it can be passed to /). (Also, you need to change x /= [] to m /= [].)

answered Jun 12, 2011 at 3:04

jwodder

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2 Comments

Visuddha Karunaratne Over a year ago

thanx jwodder! but i still can not let calculate the average i getting error ERROR file:.\PRO.hs:10 - Type error in application *** Expression : fromInteger (length m) *** Term : length m *** Type : Int *** Does not match : Integer

Thomas M. DuBuisson Over a year ago

@visuddha use fromIntegral to coerce integral types.

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