本文介绍了适用性不使用排序的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我有这种类型，基本上是Kleisli箭头:
I have this type, basically a Kleisli arrow:
{-# language DeriveFunctor #-}
data Plan m i o = Plan (i -> m o) deriving Functor
instance (Monad m) => Applicative (Plan m i) where
pure x = Plan (\_ -> pure x)
Plan f <*> Plan x = Plan (\i -> f i <*> x i)
因为它有一个Applicative实例，所以我打开ApplicativeDo并尝试使用do表示法构建一个值:
Since it has an Applicative instance, I turn on ApplicativeDo and try to build a value using do-notation:
{-# language ApplicativeDo #-}
myplan :: Plan IO () ()
myplan = do
pure ()
pure ()
它不起作用:
No instance for (Monad (Plan IO ())) arising from a do statement
有没有办法使它起作用?我正在使用GHC 8.0.1.
Is there a way to make it work? I'm using GHC 8.0.1.
推荐答案
在ApplicativeDo相关的票证之后noreferrer> GHC Trac ，看来这是一个已知问题:
After searching for ApplicativeDo-related tickets in the GHC Trac, it appears that is is a known problem:
https://ghc.haskell.org/trac/ghc/ticket /12666
明确丢弃结果是一种解决方法:
Explicitly discarding the result is a workaround:
do
_ <- pure ()
pure ()
这篇关于适用性不使用排序的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！