The basis for Object-Oriented Programming languages started in the early 1960s, and for over 30 years, it has dominated the market. Over time, many problems in the Object-Oriented Programming paradigm have been found. This article is not to describe the OOP problems, but I have quoted a text from Dijkstra. This Dijkstra quote should be the key for you to understand the OOP problems.
Functional Programming (FP) is another programming paradigm, and it exists even before Object-Oriented Programming. FP has a strong mathematical foundation based on lambda calculations.
The one thing that functional programming does well is it helps us write reliable software, and the need for a debugger almost disappears.
Functional programming uses abstract mathematics (algebra, logic); and, if you do everything pure and clean, then you can even write a mathematical proof that your source code fulfills a specific formal specification. I am like most of you, an imperative programmer. The problem is, I have learned functional programming concepts several times, but by the time I need it I have forgotten the most stuff because I do not practice it.
I have decided to learn Functional Programming in an innovative way. I will use a non-pure functional programming language, and I will create a desktop business domain application just like what we do with C#, and step by step, convert it to a clean functional style. In this way, I will not forget what I did, and I can also use it in my daily job.
In this article, I have written my experience with F#, and I hope this article can motivate you to write more functional code.
“F# is a simple and expressive programming language. It can be described as statically typed impure functional language that supports functional, imperative, and object-oriented paradigm and also several other programming styles, including data-driven, event-driven, and parallel programming.”
The Customers Demo application.
Or you can download it from
here.
Prerequisite
Visual Studio 2019 and Microsoft SQL Server Express
Step 1
I have created an F# Visual Studio project as below.
The generated Program.fs,
- open System
-
- [<EntryPoint>]
- let main argv =
-
- printfn "Hello World from F#!"
- 0
If you press F5, then you can see the Hello World from F#! in the console.
Step 2
I have added two libraries — one to access the database and the other one for the user interface based on WPF.
In the Package Manager Console,
- Install-Package SQLProvider -Version 1.1.68
- Install-Package FSharp.Desktop.UI -Version 0.7.1
Step 3
I have added the domain entity (Domain.fs) as below.
Customer class has two properties: CustomerId and Name.
- module Domain
-
- type Customer(customerId:int, name:string) =
- inherit FSharp.Desktop.UI.Model()
-
- let mutable customerId =customerId
- let mutable name = name
-
- member this.CustomerId
- with get() = customerId
- and set value =
- customerId <- value
- this.NotifyPropertyChanged "CustomerId"
-
- member this.Name
- with get() = name
- and set value =
- name <- value
- this.NotifyPropertyChanged "Name"
Step 4
I need to store/load the customer to/from the database, so I have defined my database context in the DataContext.
- module DataContext
-
- open FSharp.Data.Sql
- open Microsoft.FSharp.Collections
-
- [<Literal>]
- let connectionString = "Server=.\SQLExpress; Database=ShopDatabase;
- Trusted_Connection=true;"
-
- type Sql = SqlDataProvider<Common.DatabaseProviderTypes.MSSQLSERVER, connectionString>
-
- let DbContext = Sql.GetDataContext()
-
- let FirstCustomer =
- query {
- for customer in DbContext.Dbo.Customers do
- select customer
- } |> Seq.head;
-
- let GetNextCustomer (id) =
- query {
- for customer in DbContext.Dbo.Customers do
- where (customer.CustomerId >id)
- select customer
- } |> Seq.tryHead
-
- let GetBeforeCustomer (id) =
- query {
- for customer in DbContext.Dbo.Customers do
- where (customer.CustomerId < id)
- select customer
- } |> Seq.tryLast
Step 5
The SQL Script,
- USE master;
- GO
-
- IF DB_ID (N'ShopDatabase') IS NOT NULL
- DROP DATABASE ShopDatabase;
- GO
-
- CREATE DATABASE ShopDatabase;
- GO
-
- Use ShopDatabase
- GO
-
- CREATE TABLE Customers (
- CustomerId INT PRIMARY KEY,
- Name nvarchar(25));
- GO
-
- INSERT INTO Customers(CustomerId, Name)
- VALUES (1, 'Bassam'), (2,'Mays'), (3,'Rami'), (4,'Fadi'), (5,'Alugili');
Step 6
I have used FSharp.Desktop.UI to display the data.
„FSharp.Desktop.UI designed for building WPF applications in F#. With strong support for MVC, functional, asynchronous, and event-driven programming, it will enable you to build your solution quickly, without the need to sacrifice type system or testability.”
The main view contains the view logic. I have added in the grid the main panel and two textboxes and two buttons and some labels, and I have also registered some events like Button Click/ MouseWheel / Key Up/Down.
- module MainView
-
- open FSharp.Desktop.UI
- open System.Windows
- open System.Windows.Controls
- open System.Windows.Input
- open System.Windows.Data
- open System.Windows.Media;
-
- type NumericUpDownEvents = Up | Down
-
- type MainView() as this =
- inherit View<UpDownEvents, Domain.Customer, Window>(Window())
-
-
- do
- this.Root.Width <- 800.
- this.Root.Height <- 600.
- this.Root.WindowStartupLocation <- WindowStartupLocation.CenterScreen
- this.Root.Title <- "F# Desktop Demo Application"
- this.Root.Background <- Brushes.LightCyan
-
- let mainPanel =
- let grid = Grid(HorizontalAlignment = HorizontalAlignment.Stretch, VerticalAlignment = VerticalAlignment.Stretch, ShowGridLines = true)
- [ RowDefinition(Height= GridLength(300.)); RowDefinition(Height= GridLength(300.)) ] |> List.iter grid.RowDefinitions.Add
- [ ColumnDefinition(Width= GridLength(300.)); ColumnDefinition(Width= GridLength(300.));ColumnDefinition(Width= GridLength(200.)) ] |> List.iter grid.ColumnDefinitions.Add
- grid
-
- let idLabel = Label(Content= "Id: ", FontSize = 20., Width = 150., HorizontalAlignment = HorizontalAlignment.Left, VerticalAlignment = VerticalAlignment.Top);
-
- let userIdTextBox = TextBox(FontSize = 20., Width = 150., Height = 50., HorizontalAlignment = HorizontalAlignment.Left, VerticalAlignment = VerticalAlignment.Top, Background = Brushes.LightBlue)
-
- let userNameLabel = Label(Content= "Name: ", FontSize = 20., Width = 150., HorizontalAlignment = HorizontalAlignment.Left, VerticalAlignment = VerticalAlignment.Top)
-
- let userNameTextBox = TextBox(FontSize = 20., Width = 150., Height = 50., HorizontalAlignment = HorizontalAlignment.Left, VerticalAlignment = VerticalAlignment.Top, Background = Brushes.LightBlue)
-
- let upButton = Button(Content = "^", FontSize = 20., Width = 150., Height = 50., Background = Brushes.LightCyan)
-
- let downButton = Button(Content = "v", FontSize = 20.,Width = 150., Height = 50.,Background = Brushes.LightCyan)
-
- do
- Grid.SetRow(idLabel,0)
- Grid.SetColumn(idLabel,0)
- Grid.SetRow(userIdTextBox,0)
- Grid.SetColumn(userIdTextBox, 1)
- Grid.SetRow(userNameLabel, 1)
- Grid.SetColumn(userNameLabel, 0)
- Grid.SetRow(userNameTextBox, 1)
- Grid.SetColumn(userNameTextBox, 1)
- Grid.SetRow(upButton, 0)
- Grid.SetColumn(upButton, 2)
- Grid.SetRow(downButton, 1)
- Grid.SetColumn(downButton, 2)
-
- mainPanel.Children.Add idLabel |> ignore
- mainPanel.Children.Add userIdTextBox |> ignore
- mainPanel.Children.Add userNameLabel |> ignore
- mainPanel.Children.Add userNameTextBox |> ignore
- mainPanel.Children.Add upButton |> ignore
- mainPanel.Children.Add downButton |> ignore
-
- this.Root.Content <- mainPanel
-
-
- override this.EventStreams = [
- upButton.Click |> Observable.map (fun _ -> Up)
- downButton.Click |> Observable.map (fun _ -> Down)
- userIdTextBox.KeyUp |> Observable.choose (fun args ->
- match args.Key with
- | Key.Up -> Some Up
- | Key.Down -> Some Down
- | _ -> None
- )
- userIdTextBox.MouseWheel |> Observable.map (fun args -> if args.Delta > 0 then Up else Down)
- ]
- override this.SetBindings model =
- Binding.OfExpression
- <@
- userIdTextBox.Text <- coerce model.CustomerId
- userNameTextBox.Text <- coerce model.Name
- @>
Step 7
I have created a controller to separate the presentation logic from the business logic — the trick here in the UpDownEvents. When any button or key is pressed or you move the mouse wheel up/down in the Textbox, then I am firing in the View an up or down event. I am listing those events in the controller and retrieving the data from the database, as shown below.
- module Controller
-
- let eventHandler event (customer: Domain.Customer) =
- match event with
- | MainView.Up ->
- let nextCustomer = DataContext.GetNextCustomer customer.CustomerId
- match nextCustomer with
- | Some c -> customer.CustomerId <- c.CustomerId
- customer.Name <- c.Name
- | None -> customer.CustomerId <- 0
- customer.Name <- "Not found upper!"
-
- | MainView.Down ->
- let beforeCustomer = DataContext.GetBeforeCustomer customer.CustomerId
- match beforeCustomer with
- | Some c -> customer.CustomerId <- c.CustomerId
- customer.Name <- c.Name
- | None -> customer.CustomerId <- 0
- customer.Name <- "Not found down!"
Step 8
Finally, we have to wire the modules. I did that in the Program.fs
- open System
- open System.Windows
- open FSharp.Desktop.UI
-
- [<STAThread>]
-
-
- [<EntryPoint>]
- do
- let firstCustomer = DataContext.FirstCustomer
-
-
- let customerModel = Domain.Customer.Create(firstCustomer.CustomerId, firstCustomer.Name)
-
- let view = MainView.MainView()
-
- let controller:IController<MainView.UpDownEvents, Domain.Customer> = Controller.Create Controller.eventHandler
-
- let mvc = Mvc(customerModel, view,controller)
- use eventLoop = mvc.Start()
- Application().Run( window = view.Root) |> ignore
Your application should be like below,
Press F5 and enjoy your first F# application😊
You can click on the up/down buttons to browse the data, or you can do that by using the arrow keys up/down or the Mouse wheel.
The application is ready to use. The next question is, what we can do it better in this application?
The first problem: We have used the mutable keyword.
- let mutable customerId =customerId
- let mutable name = name
The mutable state belongs to the imperative programming style. Can we change that?
The second problem: This query throws an exception if the database does not exist or is empty. Exceptions in functional programming are not desired. Can we make it better? A tip looks to tryHead.
- let FirstCustomer = query {
- for customer in DbContext.Dbo.Customers do
- select customer
- } |> Seq.head;
Summary
Good developers must know more than one programming paradigm. One of the best choices is the functional programming paradigm. FP makes you a powerful programmer, and it allows you to solve complex problems more cleanly. Remember your salary depends on your knowledge and skills, if you can do more then the other developers, then you can negotiate a higher salary by showing that you have been valuable to the company.
In this article, we have created an F# Desktop application, and I have shown you how easily we can create a desktop application with F# for C# programmers. We have created a domain application, and in the next part, I will do more business models and business logic. Please leave a comment if you enjoyed my article.