haskell - 具有特化规则的函数依赖

Question

我想写一个specialization rewrite rule对于 Megaparsec 组合器，以便仅当输入类型为 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，它失败了:

$ 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 #-}

MonadParsec的输入流类型参数s有一个functional dependency在 Monad 参数 m 上。

class (Stream s, MonadPlus m) => MonadParsec e s m | m -> e s where

这是一个用于尝试构建的 specialize.cabal 文件。

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

如果成功，输出应如下所示:

1:1:
  |
1 | <empty line>
  | ^
expecting String generic
1:1:
  |
1 | <empty line>
  | ^
expecting ByteString specialized

建议？

Answer 1

此规则有效，但仅适用于 GHC 8.8.1，不适用于 GHC 8.6.5。

{-# LANGUAGE TypeApplications #-}

{-# RULES "combin/ByteString" forall e. forall. 
    combin @e @B.ByteString = combinByteString @e @B.ByteString
#-}

此规则适用于 GHC 8.6.5 和 8.0.2

{-# RULES "combin/ByteString"
 forall (pa :: ParsecT e B.ByteString m a).
 combin @e @B.ByteString @(ParsecT e B.ByteString m) @a pa = 
 combinByteString @e @B.ByteString @(ParsecT e B.ByteString m) @a pa
#-}