Haskell，无法匹配预期的类型

Question

我是Haskell的新手，所以我不知道出了什么问题。

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)

有人可以告诉我在哪里出现“无法匹配预期类型”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（两个元组）。

Answer 2

假设您对printBExpr和printAExpr定义都可以，那么您需要删除模式匹配项中的逗号：

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