相关疑难解决方法(0)

我看到处处Applicative可以处理副作用,但我见过的所有简单例子都是将各种东西组合在一起,如:

> (,,) <$> [1,2] <*> ["a", "b", "c"] <*> ["foo", "bar"]
[(1,"a","foo"),(1,"a","bar"),(1,"b","foo"),(1,"b","bar"),
 (1,"c","foo"),(1,"c","bar"),(2,"a","foo"),(2,"a","bar"),
 (2,"b","foo"),(2,"b","bar"),(2,"c","foo"),(2,"c","bar")]

哪个很酷,但我看不出它与副作用有何联系.我的理解是,它Applicative是一个弱单子,所以你可以处理副作用(就像你对State monad一样),但你不能重复使用前一个副作用的结果.

这是否意味着>>可以写出Applicative类似的东西

do
  print' "hello"
  print' "world"

会有意义(有print' :: a -> Applicative something)(使用适当的do-applicative扩展名).

在另一个世界,之间的区别Monad,并Applicative是Monad允许x <- ...,但Applicative没有.

然后,是作家monad,只是一个应用程序？