haskell - 返回从 monad 中提取的元素；多余的？

Question

对于所有表现良好的 Monad，以下两种 flatten 实现是否等效？

flatten1 xss = do
    xs <- xss
    x <- xs
    return x

flatten2 xss = do
    xs <- xss
    xs

Answer 1

是的，它们是相同的。它们被脱糖为

flatten1 xss =
    xss >>= \xs -> xs >>= \x -> return x

flatten2 xss = do
    xss >>= \xs -> xs

第一个相当于

xss >>= \xs -> xs >>= return

通过 Right identity monad law相当于

xss >>= \xs -> xs