[Haskell-beginners] Average of numeric list

[Daniel Fischer]

On Monday 04 April 2011 17:15:20, Nadav Chernin wrote:
> Hello,
> 
> I tried to write function mean - average of numeric list.
> 
> mean::(Fractional a)=>[a]->a
> mean a = (realToFrac (sum a)) / (realToFrac (length a))
> 
> But error occures:
> 
>               Could not deduce (Real a) from the context (Fractional b)
>               arising from a use of `realToFrac'
> To correct this function, i rewrite this function:
> 
> mean::(Real a, Fractional a)=>[a]->b
> mean a = (realToFrac (sum a)) / (realToFrac (length a))
> 
> Is there most simple way to write this function?

mean :: Fractional a => [a] -> a
mean xs = sum xs / fromIntegral (length xs)

Note however, that that's not a particularly efficient way to calculate the 
mean, since the compiler isn't smart enough to transform it into a loop 
traversing the list once (so allowing it to be garbage collected as it is 
consumed and the computation to run in constant space) and keeping the 
accumulators evaluated.

> 
> Thanks, Nadav