Question

我是Haskell的新手，我无法弄清楚出了什么问题。

data Stmt = If BExpr Stmt
      | While BExpr Stmt
      | Assign String AExpr
      deriving (Eq, Show)
printStmt :: Stmt -> String
printStmt (If e, s1)                = "if " ++ (printBExpr e) ++ " {" ++ (printStmt s1) ++ " }"
printStmt (While e, s)              = "while" ++ (printBExpr e) ++ "{" ++ (printStmt s) ++ "}"
printStmt (Assign s, e)             = s ++ ":=" ++ (printAExpr e)

有人可以告诉我，我在哪里得到“无法匹配的预期类型”错误吗？

Answer 1

从模式中删除逗号：

printStmt :: Stmt -> String
printStmt (If e s1)         = "if " ++ (printBExpr e) ++ " {" ++ (printStmt s1) ++ " }"
printStmt (While e s)       = "while" ++ (printBExpr e) ++ "{" ++ (printStmt s) ++ "}"
printStmt (Assign s e)      = s ++ ":=" ++ (printAExpr e)

(If e, s1)被解释为一对（两元组）。

Answer 2

假设您对printBExpr和printAExpr的定义没问题，则需要删除模式匹配中的逗号：

printStmt (If e s1)
printStmt (While e s)
printStmt (Assign s e)

Haskell，无法匹配预期的类型

2 个答案: