Haskell，无法匹配预期类型

Question

我是Haskell的新手，我无法弄清楚什么是错误的。

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) 

有谁可以请告诉我在哪里我得到一个“无法匹配预期类型”的错误在这里？

Answer 1

从图案卸下逗号：

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)被解释作为一对（双元组）。

Answer 2

假设你为printBExpr和printAExpr定义都OK，你需要删除你的模式匹配的逗号：

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