Haskell: Types don't match

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)?

score 13 · Accepted Answer · edited Oct 18 '11 at 10:35

13

Use fromIntegral to convert the Ints to Floats:

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

edited Oct 18 '11 at 10:35

hammar

answered Oct 18 '11 at 09:10

dave4420

score 2 · Answer 2 · answered Oct 18 '11 at 11:08

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 Ints:

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

answered Oct 18 '11 at 11:08

Landei

Isn’t `x+y+z` shorter than `sum [x,y,z]`? – Tsuyoshi Ito Oct 18 '11 at 12:38
No, but `fromIntegral $ sum [x,y,z]` is shorter than `fromIntegral x + fromIntegral y + fromIntegral z`. And it is easier to generalize... – Landei Oct 18 '11 at 12:50
1

You are answering something different from what I asked. – Tsuyoshi Ito Oct 18 '11 at 17:04

2 Answers2