haskell - ApplicativeDo 不使用测序

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)

因为它有一个 Applicative例如，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

有没有办法让它工作？我正在使用 GHC 8.0.1。

Answer 1

搜索 ApplicativeDo 后相关门票在GHC Trac ，看来这是一个已知问题:

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

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

显式丢弃结果是一种解决方法:

do
  _ <- pure ()
  pure ()