Haskell,无法匹配预期类型
我是哈斯克尔的新手,我不知道出了什么问题Haskell,无法匹配预期类型,haskell,types,match,Haskell,Types,Match,我是哈斯克尔的新手,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) ++ "
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)
有人能告诉我这里哪里出现“无法匹配预期类型”错误吗?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)
(如果e,s1)
被解释为一对(两元组)。假设您对printBExpr
和printAExpr
的定义正确,您需要删除模式匹配中的逗号:
printStmt (If e s1)
printStmt (While e s)
printStmt (Assign s e)