Am Donnerstag 23 April 2009 14:57:58 schrieb Daniel Carrera:
Hi,
I want to make a simple "average" function:
average xs = (sum xs)/(length xs)
When I try that on ghci I get the following error:
No instance for (Fractional Int) arising from a use of `/' at <interactive>:1:17-36 Possible fix: add an instance declaration for (Fractional Int) In the expression: (sum xs) / (length xs) In the definition of `average': average xs = (sum xs) / (length xs)
Ok, so 'sum' is of type (Num a => [a] -> a), length is of type ([a] -> Int) and (/) is of type (Fractional a => a -> a -> a). Can someone explain how I can "add an instance declaration" to my function so as to make all these types compatible?
Don't. (you wouldn't add the instance declaration to the function, anyway, but to your module) Try explicitly converting the length to the appropriate type: average xs = sum xs / fromIntegral (length xs) will yield a working (albeit inefficient) average :: Fractional a => [a] -> a
Thanks, Daniel.