具有特殊化规则的功能依赖性

Question

我想写一个specialization rewrite rule为Megaparsec组合器，以便仅在输入类型为ByteString时才触发规则。

{-# LANGUAGE ExplicitForAll #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeFamilies #-}

import Data.Void
import Text.Megaparsec
import qualified Data.ByteString as B

combin :: forall e s m a. (MonadParsec e s m)
    => m a
    -> m a
combin = label "String generic"

combinByteString :: forall e s m a. (MonadParsec e s m, s ~ B.ByteString)
    => m a
    -> m a
combinByteString = label "ByteString specialized"

main = do
    parseTest (combin empty :: Parsec Void String String) ""
    parseTest (combin empty :: Parsec Void B.ByteString String) ""

{-# NOINLINE combin #-}
{-# NOINLINE combinByteString #-}
{-# RULES "combin/ByteString" combin = combinByteString #-}

当我尝试构建它时失败：

$ cabal v2-run
Build profile: -w ghc-8.6.5 -O1
Main.hs:25:40: error:
    • Couldn't match type ‘s’ with ‘B.ByteString’
        arising from a functional dependency between constraints:
          ‘MonadParsec e B.ByteString m’
            arising from a use of ‘combinByteString’ at Main.hs:25:40-55
          ‘MonadParsec e s m’
            arising from the RULE "combin/ByteString" at Main.hs:25:11-55
      ‘s’ is a rigid type variable bound by
        the RULE "combin/ByteString"
        at Main.hs:25:11-55
    • In the expression: combinByteString
      When checking the transformation rule "combin/ByteString"
   |
25 | {-# RULES "combin/ByteString" combin = combinByteString #-}
s的输入流类型参数MonadParsec在MonadParsec参数functional dependency上具有Monad。 

m
这里是class (Stream s, MonadPlus m) => MonadParsec e s m | m -> e s where
文件，用于尝试构建。

specialize.cabal
如果成功，输出应如下所示：

cabal-version:       >=1.10
name:                specialize
version:             0.1.0.0
build-type:          Simple

executable specialize
  main-is:             Main.hs
  build-depends:       base >= 4
                      ,megaparsec
                      ,bytestring
  default-language:    Haskell2010
建议？

我想为Megaparsec组合器编写一个特殊化重写规则，以便仅在输入类型为ByteString时才触发该规则。 {-＃语言ExplicitForAll＃-} {-＃语言...

Answer 1

此规则有效，但仅适用于GHC 8.8.1，不适用于GHC 8.6.5。