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?

Answer 1

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 /= [].)