如何将CheckingFuelMonad与Hoopl中的状态单元组合起来？ [英] How can I combine the CheckingFuelMonad with a State monad in Hoopl?

问题描述

我正在使用 Hoopl 图书馆，并希望在重写时带上一些状态。重写函数对于使用monad是多态的，但我不知道如何将 State monad与其中一个 Fuel monads。

下面是一个简单的例子。 MyMonad 是Hoopl的 CheckingFuelMonad 和a State monad的同义词携带一面旗帜。 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 ex where 
 Bind ::（） - > Stmt O O 
 
 rewriter :: forall e x。 Stmt e x  - >事实x（） - > MyMonad（Maybe（Graph Stmt ex））
重写器（Bind（））（）= return $ do 
f<  - 获得
 if f 
然后返回$只是emptyGraph 
 else return Nothing 
  

但是这不会编译 - GHC抱怨重写的类型错误：

 无法匹配预期的类型`Graph'Block Stmt e x '
对推断类型'Maybe（gn OO）'
预期类型：CheckingFuelMonad 
（State Bool）（Maybe（Graph Stmt ex））
推断类型：CheckingFuelMonad 
（State Bool）（Maybe（Maybe（gn OO）））
  

我想要做什么可能？如何正确写入 rewrite 函数？

解决方案

浏览hoopl代码揭示了CheckingFuelMonad不是MonadTrans的一个实例，并且你不能将它作为一个实例，因为它的构造函数没有被导出。您可以 围绕CheckingFuelMonad包装StateT，如下所示： ＃ - }

import Compiler.Hoopl
import Control.Monad.State

type MyMonad = StateT Bool SimpleFuelMonad

data Stmt ex其中
Bind ::（） - > Stmt O O

rewriter :: forall e x。 Stmt e x - >事实x（） - > MyMonad（也许（Graph Stmt ex））
重写器（Bind（））（）= do
f< - 获得
如果f
则返回$只是emptyGraph
否则返回Nothing

I am using the Hoopl library and would like to carry some state around while rewriting. The rewrite functions are polymorphic regarding the monad used, but I cannot figure out how to combine a State monad with one of the library's Fuel monads.

Below is a minimal example. MyMonad is a synonym combining Hoopl's CheckingFuelMonad and a State monad carrying a flag. Stmt is just a placeholder for my intermediate language and isn't really important.

{-# 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

But this will not compile -- GHC complains that rewrite has the wrong type:

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)))

Is what I want to do possible? How can I write the rewrite function correctly?

解决方案

A browse through hoopl code reveals that CheckingFuelMonad isn't an instance of MonadTrans, and you can't make it one, since its constructors are not exported. You can however wrap a StateT around CheckingFuelMonad, like so:

{-# 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

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