Haskell：类型不匹配

Question

I just tried to write a simple function to calculate the average of the input-Ints:

avg :: Int -> Int -> Int -> Float
avg x y z = (x+y+z)/3

When I exchange the signature to

avg :: Float -> Float -> Float -> Float

it works fine, but with the one above I get the following error message:

Couldn't match expected type 'Float' with actual type 'Int'.

Which possibilites do I have to use the first signature, which accepts Ints (only)?

Answer 1

Use fromIntegral to convert the Int s to Float s:

avg :: Int -> Int -> Int -> Float
avg x y z = (fromIntegral x + fromIntegral y + fromIntegral z) / 3

Answer 2

Shorter:

avg :: Int -> Int -> Int -> Float
avg x y z = (fromIntegral $ sum [x,y,z]) / 3.0

Or you generalize this for a list of Int s:

avg :: [Int] -> Float
avg xs = (fromIntegral $ sum xs) / (fromIntegral $ length xs)