如何使用Expr使用參數

Question

let useConnection expr = 
    let Expr(conn : MySqlConnection) = 
     try 
      try 
       conn.Open() 
      with 
      | :? MySqlException as ex 
       -> printfn "Exception! %s" ex.Message 
      expr(conn) 
     finally 
      try 
       conn.Close() |> ignore 
      with 
      | :? MySqlException as ex 
       -> printfn "Exception! %s" ex.Message 
    using (new MySqlConnection(ConnectionString = 
      "server  = " + MySQLServer + "; 
      uid   = " + MySQLUID + "; 
      pwd   = " + MySQLPW + "; 
      database = " + MySQLDB + "; 
      Charset=utf8;")) Expr 

member x.reportToDB (msg:string) = 
    useConnection // <--- SO HERE I WANT TO KNOW WHAT IS conn 
     (let cmd = new MySqlCommand(Connection = conn) 
     cmd.CommandText <- ("insert into "+MySQLTable+"(system,dt,logMessage);") 
     ignore <| cmd.Parameters.AddWithValue("?system", Net.Dns.GetHostName()) 
     ignore <| cmd.Parameters.AddWithValue("?dt", DateTime.Now.ToString()) 
     ignore <| cmd.Parameters.AddWithValue("?logMessage", msg) 
     try 
      try 
       cmd.ExecuteNonQuery() |> ignore 
      with 
      | :? MySqlException as ex when ex.Message.Contains("Duplicate entry") 
       -> printfn "MySQL Duplicate entry Exception: discarding the data set! %s" ex.Message 
        printfn "" 
      | :? MySqlException as ex 
       -> printfn "MySQL Exception, requeing data set and trying again later! %s" ex.Message 
        reraise() 
     with 
     | :? MySqlException as ex 
      -> printfn "Exception! %s" ex.Message) 

這很難解釋，但我想使用委託conn從useConnection到x.reportToDB，我該怎麼做？

謝謝。

@蒂姆羅賓遜，是的，我不知道康恩那裏，那是我想解決的問題， 爲什麼你認爲拉姆達在這裏是個壞主意？

Answer 1

useConnection似乎想要一個功能，需要一個MySqlConnection。它爲此功能提供所需的連接對象。

修復的方法是：

useConnection (fun conn (* here's your connection *) -> 
    let cmd = new MySqlCommand(Connection = conn) 
    // etc. 

編輯：這或許更加清楚加入useConnection功能類型註釋：

let useConnection (expr : MySqlConnection -> 'a) : 'a = 
    let Expr(conn : MySqlConnection) : 'a = 
    // etc.