[Haskell-beginners] Double's

[Thomas Friedrich]

Hi everybody,

please have a look at the following lines of code:

x :: [Double]
x = [2,4,6,8,10]

p :: [Double]
p = [0.05, 0.2, 0.35, 0.3, 0.1]

expectation :: Double
expectation = sum $ zipWith (*) x p

variance :: Double
variance = sum $ zipWith f x p
    where f i p = (i - expectation)^2 * p


when I load this into ghci I get the following:

*Main> expectation
6.3999999999999995
*Main> expectation == 6.4
False
*Main> variance
4.24
*Main>

Why does 'expectation' get evaluated to 6.39999... instead of 6.4??  
This is very strange.  I have another example which is even more 
annoying:  Have a look at the following code:

import qualified Data.Map as M

newtype Distribution a = Distribution (M.Map a Double)

isDistribution :: Distribution a -> Bool
isDistribution (Distribution d) = sum (M.elems d) == 1

uniform :: Ord a => [a] -> Distribution a
uniform xs = Distribution (M.fromList (map go xs))
    where go i = (i, 1 / fromIntegral (length xs))


When I load this into ghci I get the following:

*Probability> isDistribution $ uniform [1..6]
False
*Probability> (\(Distribution d) -> sum (M.elems d)) $ uniform [1..6]
0.9999999999999999
*Probability>

I mean ... hey, what is going on here?  Is there any way of getting 
around this?

Cheers,
Thomas