Question

I’m trying to grab data from a MySQL database.

Approach 2 – apply/map style

I’m using the MySQL ADO Reference to try to build this system. In particular, the example found at 21.2.3.1.7.

(using a pseudo code)

let table = build_sequence(query.read) 

Where query.read returns a row in the table(Or rather, a list of elements that happen to be a row in the table). And the table variable is a list of lists that will represent a table returned from the query.

I’ve stared at the code given below, and it’s syntax is over my head, I’m afraid.

Approach 1 – looping.

Problem 1: It’s inelegant, requiring a mutable.

Problem 2: This just feels wrong, based on my prior experience with Prolog & Lisp. There’s gotta be a more…functional way to do this.

I’m not sure where to begin though. Comments & thoughts?

let reader : MySql.Data.MySqlClient.MySqlDataReader = command.ExecuteReader()  let arr = []  let mutable rowIter = 0 let readingLoop() =     while(reader.Read()) do         rowIter = rowIter + 1         for i = 0 to reader.FieldCount do              //set arr[someiterator, i] = reader.GetValue[i].ToString()) 

Answer 1

The Seq type has a neat function for handling database cursors called generate_using (see F# Manual and the Data Access chapter in Foundations of F#). This is a higher order function that takes one function to open the cursor and another (called repeatedly) to process records from the cursor. Here is some code that uses generate_using to execute a sql query:

let openConnection (connectionName : string) =     let connectionSetting = ConfigurationManager.ConnectionStrings.Item(connectionName)     let connectionString = connectionSetting.ConnectionString     let connection = new OracleConnection(connectionString)     connection.Open()     connection  let generator<'a> (reader : IDataReader) =     if reader.Read() then         let t = typeof<'a>         let props = t.GetProperties()         let types = props                     |> Seq.map (fun x -> x.PropertyType)                     |> Seq.to_array         let cstr = t.GetConstructor(types)         let values = Array.create reader.FieldCount (new obj())         reader.GetValues(values) |> ignore         let values = values                      |> Array.map (fun x -> match x with | :? DBNull -> null | _ -> x)         Some (cstr.Invoke(values) :?> 'a)     else         None  let executeSqlReader<'a> (connectionName : string) (sql : string) : 'a list =             let connection = openConnection connectionName      let opener() =          let command = connection.CreateCommand(CommandText = sql, CommandType = CommandType.Text)         command.ExecuteReader()      let result = Seq.to_list(Seq.generate_using opener generator)              connection.Close()     connection.Dispose()     result 

For example to list all the tables in an Oracle database we need to define a column definition type and invoke executeSqlReader as follows:

type ColumnDefinition = {     TableName : string;     ColumnName : string;     DataType : string;     DataLength : decimal;                 }  let tableList = executeSqlReader<ColumnDefinition>     'MyDatabase'     'SELECT t.table_name, column_name, data_type, data_length FROM USER_TABLES t, USER_TAB_COLUMNS c where t.TABLE_NAME = c.table_name order by t.table_name, c.COLUMN_NAME'