Applicative不用于测序

Question

我有这种类型,基本上是Kleisli箭头:

{-# 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)

由于它有一个Applicative实例,我打开ApplicativeDo并尝试使用do-notation构建一个值:

{-# 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

有没有办法让它发挥作用？我正在使用GHC 8.0.1.

Answer 1

在GHC Trac中搜索ApplicativeDo相关票证后,似乎是一个已知问题:

• https://ghc.haskell.org/trac/ghc/ticket/10892

• https://ghc.haskell.org/trac/ghc/ticket/12666

明确丢弃结果是一种解决方法:

do
  _ <- pure ()
  pure ()