Question

我正在使用Hoopl库，并希望在重写时携带一些状态。重写函数对于使用的monad是多态的，但我无法弄清楚如何将State monad与库的Fuel monad组合。

以下是一个最小的例子。 MyMonad是Hoopl的CheckingFuelMonad和携带旗帜的State monad的同义词。 Stmt只是我的中间语言的占位符，并不重要。

{-# LANGUAGE GADTs, RankNTypes #-}

import Compiler.Hoopl
import Control.Monad.State

type MyMonad = CheckingFuelMonad (State Bool)

data Stmt e x where
  Bind :: () -> Stmt O O

rewriter :: forall e x. Stmt e x -> Fact x () -> MyMonad (Maybe (Graph Stmt e x))
rewriter (Bind ()) () = return $ do
  f <- get
  if f 
   then return $ Just emptyGraph
   else return Nothing

但这不会编译 - GHC抱怨rewrite的类型错误：

Couldn't match expected type `Graph' Block Stmt e x'
       against inferred type `Maybe (g n O O)'
  Expected type: CheckingFuelMonad
                   (State Bool) (Maybe (Graph Stmt e x))
  Inferred type: CheckingFuelMonad
                   (State Bool) (Maybe (Maybe (g n O O)))

我想做的是什么？如何正确编写rewrite函数？

Answer 1

浏览hoopl代码显示，CheckingFuelMonad不是MonadTrans的实例，并且您不能将其设置为一个，因为它的构造函数不会导出。你可以然后在CheckingFuelMonad周围包装一个StateT，如下所示：

{-# LANGUAGE GADTs, RankNTypes #-}

import Compiler.Hoopl
import Control.Monad.State

type MyMonad = StateT Bool SimpleFuelMonad

data Stmt e x where
  Bind :: () -> Stmt O O

rewriter :: forall e x. Stmt e x -> Fact x () -> MyMonad (Maybe (Graph Stmt e x))
rewriter (Bind ()) () = do
  f <- get
  if f
   then return $ Just emptyGraph
   else return Nothing

Answer 2

那么，您当前错误的直接原因很简单。如果f为真，最终表达式是什么？如果我们接受这个：

rewriter :: forall e x. Stmt e x -> Fact x () -> MyMonad (Maybe (Graph Stmt e x))
rewriter (Bind ()) () = return $ do
  f <- get
  if f 
   then return $ Just emptyGraph
   else return Nothing

...除去我们得到的True分支以外的所有内容：

rewriter :: forall e x. Stmt e x -> Fact x () -> MyMonad (Maybe (Graph Stmt e x))
rewriter (Bind ()) () = return $ do
  return $ Just emptyGraph

...简化为：

rewriter :: forall e x. Stmt e x -> Fact x () -> MyMonad (Maybe (Graph Stmt e x))
rewriter (Bind ()) () = return $ return $ Just emptyGraph

return $ return $ Just emptyGraph的类型是什么？

(Monad m1, Monad m2, GraphRep g) => m1 (m2 (Maybe (g n O O)))

换句话说，你在那里有一个额外的return。 (Monad m) => CheckingFuelMonad m本身就是Monad，即使CheckingFuelMonad似乎没有被定义为monad变换器，所以你只有一个monad层到return。

如何将CheckingFuelMonad与Hoopl中的State monad结合使用？

2 个答案: