Daniel Fischer wrote:
Try explicitly converting the length to the appropriate type:
average xs = sum xs / fromIntegral (length xs)
Thanks. Could you help me understand what's happening? 1. length returns Int. 2. sum returns Num. 3. (/) wants Fractional. It looks like (/) is happy with Num but doesn't like Int. This surprises me. I would have thought that Fractional is a kind of Num and Int is a kind of Fractional, so a function that expects Fractional would be happy with an Int but maybe not with a Num. But clearly that's not the way it works. 'fromIntegral' converts Int to Num. So obviously, Num is good and Int is bad. But I don't really get why.
will yield a working (albeit inefficient) average :: Fractional a => [a] -> a
Why is it inefficient? How would you make it efficient? Thanks for the help. Cheers, Daniel.