Question

我对Haskell还是陌生的。我试图通过将Either视为monad将State monad与错误传播结合起来。我想以不需要显式处理状态或错误的方式来递归抽象语法树（例如，用于在语句和表达式上编写解释器）。我的印象是，最简单的方法是使用ExceptT monad变压器。这是我编译的示例代码：

import Control.Monad.Except
import Control.Monad.State
import qualified Data.Map.Strict as M

-- simple expression language supporting crude let bindings
data Exp = Lit Int | Var String
         | Add (Exp, Exp) | Let (String, Exp, Exp) deriving Show

okExp =  -- let x = 2 in let y = x + 3 in x + y -- evaluate to 7
    Let ("x", Lit 2,
             Let ("y", Add (Var "x", Lit 3),
                      Add (Var "x", Var "y")))
badExp = Var "x"  -- error: x is not defined

type St = M.Map String Int
initialState :: St
initialState = M.empty

type EvalMonad = ExceptT String (State St)

evalExp :: Exp -> EvalMonad Int
evalExp (Lit n) = return n
evalExp (Var v) = do
    mp <- lift get
    case M.lookup v mp of
        Just i -> return i
        Nothing -> throwError (v ++ " not found")
evalExp (Add (a, b)) = do
    x <- evalExp a
    y <- evalExp b
    return (x + y)

我希望在简单的示例（例如，okExp，badExp）上运行evalExp。我不确定三件事：

如何将初始状态纳入计算？
如何使用runExceptT提取结果？
（更为笼统）：这是解决此问题的“正确”方法吗？

Answer 1

看起来像一个不错的开始！这是一个小示例，展示了如何在ghci中一起使用runExceptT和runState：

> runState (runExceptT (evalExp (Add (Lit 3, Lit 4)))) initialState
(Right 7,fromList [])
> runState (runExceptT (evalExp (Add (Lit 3, Var "x")))) initialState
(Left "x not found",fromList [])

将State monad与任何一种错误传播方式结合起来

1 个答案: