哈斯克尔可扩展效果:另一种效果中的效果

Question

我正在尝试使用extensible-skeleton包。将一个效果叠加到另一个效果中会导致编译错误。我尝试了一些其他语言扩展和类型注释，但无法删除这些错误。如何解决这些错误？

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
module Main where
import Data.Extensible
import Data.Extensible.Effect
import Data.Type.Equality
type In0 effs = Lookup effs "io" IO
type In1 effs = Lookup effs "reader-float" (ReaderEff Double)
run0 :: forall a. Eff '["io" >: IO] a -> IO a
run0 = retractEff
run1 :: forall effs a. Double -> Eff (("reader-float" >: ReaderEff Double) ': effs) a -> Eff effs a
run1 x = peelEff0 pure $ Refl k -> k x
lift0 :: forall effs a. In0 effs => IO a -> Eff effs a
lift0 = liftEff (Proxy :: Proxy "io")
ask1 :: forall effs. In1 effs => Eff effs Double
ask1 = askEff (Proxy :: Proxy "reader-float")
eff0 :: forall effs. In0 effs => Eff effs ()
eff0 = do
lift0 $ print "eff0"
run1 2.5 eff1
eff1 :: forall effs. (In0 effs, In1 effs) => Eff effs ()
eff1 = do
x <- ask1
lift0 $ print "eff1"
lift0 $ print (floor x)
main :: IO ()
main = do
run0 eff0

编译时的错误：

[1 of 2] Compiling Main
app/Main.hs:32:12: error:
• Couldn't match type ‘membership-0:Type.Membership.Internal.Elaborate
"io" (membership-0:Type.Membership.Internal.FindAssoc 1 "io" effs)’
with ‘'membership-0:Type.Membership.Internal.Expecting (n0 ':> IO)’
arising from a use of ‘eff1’
The type variable ‘n0’ is ambiguous
• In the second argument of ‘run1’, namely ‘eff1’
In a stmt of a 'do' block: run1 2.5 eff1
In the expression:
do lift0 $ print "eff0"
run1 2.5 eff1
• Relevant bindings include
eff0 :: Eff effs () (bound at app/Main.hs:30:1)
|
32 |   run1 2.5 eff1
|            ^^^^

怎么了？

Answer 1

问题似乎是run1(顺便说一句，它已经作为runReaderEff可用(接受最外层为"reader-float"的效果，但您尝试run1的效果eff1只满足In1 effs，这保证了"reader-float"在某个地方，但不一定在最外层。

这反映了这样一个事实，即不能运行隐藏在堆栈中的任意层。您需要从外向内运行图层。

有效的方法是为eff1构建一个显式效果堆栈，然后向上投射：

eff0 :: forall effs. In0 effs => Eff effs ()
eff0 = do
lift0 $ print "eff0"
castEff $ run1 2.5 (eff1 :: Eff '["reader-float" :> ReaderEff Double, "io" :> IO] ())

您可能想将"reader-float"预先设置为eff0的effs，因此写下：

{-# LANGUAGE ScopedTypeVariables #-}
eff0 :: forall effs. In0 effs => Eff effs ()
eff0 = do
lift0 $ print "eff0"
castEff $ run1 2.5 (eff1 :: Eff (("reader-float" :> ReaderEff Double) ': effs) ())

但这行不通。但是，如果您仔细考虑一下，除了特定的["reader-float", "io"]堆栈之外，没有理由在这里运行eff1。

记住，在eff0和eff1的类型签名中使用通用effs的原因是为了让它们在任何具有所需层的效果堆栈中使用；一般的类型签名是为了调用方的利益。在这种情况下，eff0正在调用eff1，并且它没有理由向eff1提供除了它需要的层、它在本地构建的新的"reader-float"层以及"；io"；层，它将其投射到自己的通用CCD_ 22环境中。effs中没有eff0或eff1感兴趣的其他内容，因此不需要将effs从eff0向下传播到eff1。