相关疑难解决方法(0)

我有一些代码来评估原始程序.程序是一个语句列表(表达式,块,返回语句).评估结果是最后评估的表达式.评估员也应妥善处理return陈述(即首次出现后停止评估return).

为了实现这个逻辑,我传递了特殊的回调函数(NextStep),它在当前语句之后进行下一个评估步骤.处理return语句时我不调用下一步:

data Statement = 
      Expr Int
    | Block [Statement]
    | Return Int
    deriving (Show, Eq)

data Value = 
      Undefined
    | Value Int
    deriving (Show, Eq)

type NextStep = Value -> Value

evalStmt :: Statement -> NextStep -> Value
evalStmt (Expr val) next = 
    let res = Value val
    in next res
evalStmt (Block stmts) next = evalBlock stmts next
evalStmt (Return val) next = Value val

evalBlock :: [Statement] -> NextStep -> Value …