如何将解释器带入 IO monad？

Question

我的问题与in this answer

中编写的简单解释器有关

如何将 IO 功能添加到 this interpreter（第一个非 monadic 版本）？我的意思只是添加一个使用 putStrLn 的语句。我还不太精通 Haskell，但我猜你可以以某种方式组合 IO monad。有人能给我指出正确的方向吗？

data Stmt
  = Var := Exp                                   
  | While Exp Stmt                                               
  | Seq [Stmt]      
  | Print Exp       -- a print statement

Answer 1

你可以让你的解释器在 IO monad 中有一个结果：

exec :: Stmt -> Store -> IO Store

现在 exec 函数可以在 IO 中做任何事情。现有操作将需要相应修改。

Answer 2

我已经使用 Print 语句扩展了您的解释器，该语句将表达式打印到控制台。 exec 和运行的签名必须相应地扩展：

infixl 6 :+:, :-:
infixl 7 :*:, :/:

data Exp
    = C Int        -- constant                                                     
    | V Var        -- variable                                                     
    | Exp :+: Exp  -- addition                                                     
    | Exp :-: Exp  -- subtraction                                                  
    | Exp :*: Exp  -- multiplication                                               
    | Exp :/: Exp  -- division
    deriving (Show)

infix 1 :=

data Stmt
    = Var := Exp      -- assignment                                                
    | While Exp Stmt  -- loop                                                      
    | Seq [Stmt]      -- sequence
    | Print Exp


type Prog = Stmt

type Var = String
type Val = Int
type Store = [(Var, Val)]

eval :: Exp -> Store -> Val
eval (C n) r       = n
eval (V x) r       = case lookup x r of
                    Nothing -> error ("unbound variable `" ++ x ++ "'")
                    Just v  -> v
eval (e1 :+: e2) r = eval e1 r + eval e2 r
eval (e1 :-: e2) r = eval e1 r - eval e2 r
eval (e1 :*: e2) r = eval e1 r * eval e2 r
eval (e1 :/: e2) r = eval e1 r `div` eval e2 r

exec :: Stmt -> Store -> IO Store
exec (x := e) r                    = return $ (x, eval e r) : r
exec (While e s) r | eval e r /= 0 = exec (Seq [s, While e s]) r
                | otherwise     = return r
exec (Seq []) r                    = return r
exec (Seq (s : ss)) r              = do
                                        r' <- exec s r
                                        exec (Seq ss) r'
exec (Print x) r                   = print (eval x r) >> return r

run :: Prog -> Store -> IO Store
run p r = nubBy ((==) `on` fst) <$> (exec p r)

fib :: Prog
fib = Seq
[ "x" := C 0
, "y" := C 1
, While (V "n") $ Seq
    [ "z" := V "x" :+: V "y"
    , "x" := V "y"
    , "y" := V "z"
    , "n" := V "n" :-: C 1
    , Print (V "x")
    ]
]

main = lookup "x" <$> run fib [("n", 25)]

如何将解释器带入 IO monad？

How to bring an interpreter to the IO monad?

interpreter

haskell

language-design