F# Beginner: retrieving an array of data from a server - mysql

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"

Answer 2

It can be hard to work with imperative APIs in a non-imperative way. I don't have MySql handy, but I made an approxmiation, hopefully this will provide inspiration. Seq.unfold is a function people find pretty awesome once they grok it. List.init (or Array.init) are also handy for initializing known-size data structures without using mutables.
#light
type ThingLikeSqlReader() =
let mutable rowNum = 0
member this.Read() =
if rowNum > 3 then
false
else
rowNum <- rowNum + 1
true
member this.FieldCount = 5
member this.GetValue(i) = i + 1
let reader = new ThingLikeSqlReader()
let data = reader |> Seq.unfold (fun (reader : ThingLikeSqlReader) ->
if reader.Read() then
Some (List.init reader.FieldCount (fun i -> reader.GetValue(i)), reader)
else
None) |> Seq.to_list
printfn "%A" data