Hi,

You can define it, but in practice there is no instance of "a" that satisfies both constraints: Integral a and Fractional a
> meanList ([1,2,3] :: [Int])

<interactive>:4:1: error:
    • No instance for (Fractional Int) arising from a use of ‘meanList’

> meanList ([1,2,3] :: [Float])

<interactive>:5:1: error:
    • No instance for (Integral Float) arising from a use of ‘meanList’

What you probably want is:
meanList :: (Integral a, Fractional b) => [a] -> b
meanList xs = fromIntegral (sumList xs) / fromIntegral (lengthList xs)

Where we convert from the integral type "a" to the fractional type "b" before performing the division.

> meanList ([1,2,3] :: [Int])
2.0

Cheers
Sylvain

Hi, can someone explain to me why i cannot define meanList as:

meanList :: (Integral a, Fractional b) => [a] -> a

meanList xs = (sumList xs) / (lengthList xs)

I want to restrict the function to only accept lists like [1,2,3] and return answer 2.0

sumList :: (Num a) => [a] -> a

sumList [] = 0

sumList (x:xs) = x + (sumList xs)

lengthList :: (Num a) => [t] -> a

lengthList [] = 0

lengthList (_:xs) = 1 + (lengthList xs)

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