Question

我已将以下重写规则添加到管道而没有问题：

{-# RULES "ConduitM: lift x >>= f" forall m f.
    lift m >>= f = ConduitM (PipeM (liftM (unConduitM . f) m))
  #-}

我正在尝试为liftIO添加类似的重写规则

{-# RULES "ConduitM: liftIO x >>= f" forall m f.
    liftIO m >>= f = ConduitM (PipeM (liftM (unConduitM . f) (liftIO m)))
  #-}

但是，当我尝试这样做时，我从GHC收到以下错误消息：

Data/Conduit/Internal/Conduit.hs:1025:84:
    Could not deduce (Monad m) arising from a use of ‘liftM’
    from the context (Monad (ConduitM i o m), MonadIO (ConduitM i o m))
      bound by the RULE "ConduitM: liftIO x >>= f"
      at Data/Conduit/Internal/Conduit.hs:1025:11-118
    Possible fix:
      add (Monad m) to the context of the RULE "ConduitM: liftIO x >>= f"
    In the first argument of ‘PipeM’, namely
      ‘(liftM (unConduitM . f) (liftIO m))’
    In the first argument of ‘ConduitM’, namely
      ‘(PipeM (liftM (unConduitM . f) (liftIO m)))’
    In the expression:
      ConduitM (PipeM (liftM (unConduitM . f) (liftIO m)))

Data/Conduit/Internal/Conduit.hs:1025:108:
    Could not deduce (MonadIO m) arising from a use of ‘liftIO’
    from the context (Monad (ConduitM i o m), MonadIO (ConduitM i o m))
      bound by the RULE "ConduitM: liftIO x >>= f"
      at Data/Conduit/Internal/Conduit.hs:1025:11-118
    Possible fix:
      add (MonadIO m) to the context of
        the RULE "ConduitM: liftIO x >>= f"
    In the second argument of ‘liftM’, namely ‘(liftIO m)’
    In the first argument of ‘PipeM’, namely
      ‘(liftM (unConduitM . f) (liftIO m))’
    In the first argument of ‘ConduitM’, namely
      ‘(PipeM (liftM (unConduitM . f) (liftIO m)))’

我不知道任何语法可以让我为重写规则指定这样的上下文。有没有办法实现这个目标？

Answer 1

您可以在规则中指定具有约束的参数类型，例如

{-# RULES "ConduitM: liftIO x >>= f" forall m (f :: (Monad n, MonadIO n) => CounduitM i o n r).
    liftIO m >>= f = ConduitM (PipeM (liftM (unConduitM . f) (liftIO m)))
  #-}

（我没有对它进行过测试，因为我没有安装相关的软件包，但据我了解所涉及的类型，我认为应该可行。）

Answer 2

我试图找出如何在重写规则中添加约束的类似效果。使用相同的语法我能够编译GHC，但显然，在这种情况下，重写规则将永远不会触发。

这是一个简单的例子：

#!/usr/bin/env stack
-- stack --resolver lts-7.14 exec -- ghc -O -ddump-rule-firings
module Main where

import Prelude as P
import System.Environment (getArgs)


class Num e => Power e where
  (^:) :: Integral a => e -> a -> e

instance Power Double where
  (^:) x y = go 0 1 where
    go n acc | n < y = go (n+1) (acc*x)
             | n > y = go (n-1) (acc/x)
             | otherwise = acc

main :: IO ()
main = do
  [xStr] <- getArgs
  let x = read xStr :: Double
  print (x ^ 24)

{-# RULES
 "1. Test ^" forall (x :: Power x => x) n. x ^ n = x ^: n;
 "2. Test ^" forall x n. (x :: Double) ^ n = x ^: n 
 #-}

即使删除了第二条规则，也不会触发第一条规则。这是一个类似的SO问题，它解答了为什么它不会触发：GHC rewrite rule specialising a function for a type class

具有类约束的GHC重写规则

2 个答案: