相关疑难解决方法(0)

我已经定义了以下数据类型:

data Probability a = PD { mass :: [(a, Ratio Int)] } 

现在我想写一下它是一个实例Functor:

collect :: (Eq a, Num b) => [(a, b)] -> [(a, b)]
collect al = map collect' keys where
    keys = nub $ map fst al
    collect' k = (k, sum (map snd (matches k)))
    matches k = filter ((==) k . fst) al

instance (Eq a) => Functor (Probability a) where
    fmap f p = PD (collect $ map (first f) (mass p)) …