如何在遍历抽象语法树时使用Reader Monad

Question

我有一个由Parsec制作的haskell抽象语法树.我希望能够在同时遍历它时查询其结构,以便将其转换为中间代码.例如,我需要知道我的AST的任何给定函数有多少参数才能进行这种转换.我目前正在做的是将AST传递给每个函数,所以每当我需要进行查找时我都可以调用它,并且我在另一个文件中有辅助函数来为我执行查找.这污染了我的类型签名.特别是当我开始添加更多东西,如累加器.

而不是将AST传递给我听过的每个函数,这对于Reader Monad(对于不改变的状态,AST)和状态Monad(对于确实改变的状态,累加器)来说是一个好工作.

如何从IO monad(gulp)中取出ast 并在Reader Monad中使用它来进行全局查找？

main = do
  putStrLn "Please enter the name of your jack file (i.e. Main)"
  fileName <- getLine
  file <- readFile (fileName++".jack")
  let ast = parseString file
  writeFile (fileName++".xml") (toClass ast) --I need to query this globally
  putStrLn $  "Completed Parsing, " ++ fileName ++ ".vm created..."

type VM = String

toClass :: Jack -> VM
toClass c = case c of
   (Class ident decs) ->
     toDecs decs 

toDecs ::[Declaration] -> VM -- I don't want to add the ast in every function arg...
toDecs [] = ""
toDecs (x:xs) = case x of
  (SubDec keyword typ subname params subbody) ->
    case keyword of
      "constructor" -> --use the above ast to query the # of local variables here...
    toSubBody subbody ++
    toDecs xs
  otherwise -> []

关于Reader Monad进展的更新:我已经将上面的例子变成了这样的东西:(见下文).但现在我想知道由于所有这些字符串输出的积累,我应该使用作家Monad吗？如果是这样,我该怎么做才能写出这两个？ReaderT应该封装作者吗？或相反亦然？我应该制作一个只接受Reader和Writer而不试图将它们组成Monad Transformer的类型吗？

main = do
  putStrLn "Please enter the name of your jack file (i.e. Main)"
  fileName <- getLine
  file <- readFile (fileName++".jack")
  writeFile (fileName++".xml")  (runReader toClass $ parseString file)
  putStrLn $  "Completed Parsing, " ++ fileName ++ ".xml created..."

toClass = do   
  env <- ask
  case env of Class ident decs -> return $ toDecs decs env

toDecs [] = return ""
toDecs ((SubDec keyword typ subname params subbody):xs) = do
  env <- ask
  res <- (case keyword of
            "method" -> do return "push this 0\n"
            "constructor" -> do return "pop pointer 0\nMemory.alloc 1\n"
            otherwise -> do return "")
  return $ res ++ toSubBody subbody env ++ toDecs xs env
toDecs (_:xs) = do
  decs <- ask
  return $ toDecs xs decs

toSubBody (SubBodyStatement states) = do
    return $ toStatement states
toSubBody (SubBody _ states) = do
    return $ toStatement states

http://hpaste.org/83595 - 申报

Answer 1

如果不了解更多有关Jack和Declaration类型的信息，就很难了解如何将其转换为 Reader monad。如果想法是在对象处于ast :: Jack范围内的同时对某些对象执行“映射”或“折叠”，您可以编写

f :: [Declaration] -> Reader Jack [Something]
f decls = mapM go decls where 
  go :: Declaration -> Reader Jack Something
  go (SubDec keyword typ subname params subbody) =
    case keyword of
      "constructor" -> do
        ast <- ask
        return (doSomething subbody ast)

然后在您的astas上下文中执行它runReader (f decls) ast。