Purescript Eff Monad:使用非本机计算效果

Question

我想要能够写作

x :: Eff (reader :: Reader Int, maybe :: Maybe) Int
x = do
  config <- ask -- configuration from (Reader Int) monad
  just config -- using (Maybe) Monad

runX :: Int
runX = runPure (runMaybe doIfNothing (runReader 6 x)) -- outputs: 6

使用Eff Monad

使用Eff可以做到这一点吗

如果不使用Eff，我们如何才能使其正常工作

Answer 1

您可以在Eff之上使用MaybeT和ReaderT monad转换器，但您不能以上面编写的方式匹配这两个转换器：

import Prelude
import Data.Maybe
import Control.Monad.Eff
import Control.Monad.Eff.Class
import Control.Monad.Eff.Console
import Control.Monad.Maybe.Trans
import Control.Monad.Reader.Trans

x :: ReaderT Int (MaybeT (Eff (console :: CONSOLE))) Int
x = do
  liftEff (log "Asking...")
  config <- ask
  pure config

main :: Eff (console :: CONSOLE) (Maybe Int)
main = runMaybeT (runReaderT x 6)