F# is an open-source programming language initially developed at Microsoft Research to extend .NET, Microsoft’s set of tools, libraries, and languages to build applications and services. Besides its remarkably concise syntax, F# supports multiple paradigms, meaning that it can do different types of code structuring, though it was primarily designed to take advantage of the functional programming approach.
Adopting a specific paradigm, or a style of code, determines the way we will think and organize our programming problem solving. With an imperative approach, the design model used in languages like C++ or Java, a developer describes step-by-step how the computer must accomplish a task. It’s about writing a sequence of statements that will change memory states at the program’s execution. This works fine until we encounter some irregular situations. Consider a shared object for instance, which is used by multiple applications simultaneously. We might want to read its value at the same time that another component is modifying it. These are concurrent actions upon a memory location that can produce data inconsistency and undefined behavior.
In functional code design, we prevent this kind of problem by minimizing the use of mutable states, or states that can change after we make them. Function is the keyword here, referring to mathematical transformations on some information provided as arguments. A functional code expresses what the program is by composing the solution as a set of functions to be executed. Typically, we build up layers of logic using functions that can return another function or take other functions as inputs.
Functional programming with F# brings a number of benefits:
Since F# is cross-platform, maintaining a similar execution model behavior through different operating systems is essential. .NET achieves this by means of a runtime. A runtime system is a piece of software that orchestrates the execution of a program written with a specific programming language, handling interfacing with the operating system and memory management, among other things.
There are actually two .NET runtime implementations available on Linux: .NET Core and Mono. Historically, .NET only worked on Windows. In those days, one could resort to the community Mono project to run .NET applications on other platforms like Linux and macOS. Microsoft then launched .NET Core, a faster, modular subset of the original .NET framework, to target multiple platforms.
At the time of this tutorial’s publication, they both can be used for building web applications or command line utilities. That said, .NET Core does not ship models to create GUI desktop applications on Linux and macOS, while Mono is the only one to support mobile and gaming platforms. It is important to know these differences since the runtime you pick will shape the programs you will build. You could also choose to have both .NET Core and Mono installed in order to account for all use cases and to make a more productive stack.
In this tutorial, you will set up an F# programming environment on Ubuntu 18.04 using both .NET Core and Mono runtimes. You will then write some code examples to test and review build and compile methods.
Microsoft provides the .NET Core Software Development Kit (SDK) for F# developers. A Software Development Kit is a set of programming tools that allows programmers to produce specialized applications and adapt them to various operating systems. It traditionally includes a text editor, languages support, a runtime, and a compiler, among other components. In this step, you are going to install this SDK. But first, you will register the Microsoft repository and fetch some dependencies.
You’ll be completing the installation and setup on the command line, which is a non-graphical way to interact with your computer. That is, instead of clicking on buttons, you’ll be typing in text and receiving feedback from your computer through text as well.
The command line, also known as a shell or terminal, can help modify and automate many of the tasks you do on a computer every day, and is an essential tool for software developers. There are many terminal commands to learn that can enable you to do more powerful things. For more information about the command line, check out the Introduction to the Linux Terminal tutorial.
On Ubuntu 18.04, you can find the Terminal application by clicking on the Ubuntu icon in the upper-left hand corner of your screen and typing
terminal into the search bar. Click on the Terminal application icon to open it. Alternatively, you can hit the
T keys on the keyboard at the same time to open the Terminal application automatically.
Once you have opened the terminal, use the
wget command to download a package containing some required files, the Microsoft repository configurations, and a key for server communication.
- wget -q https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb
Now, add the Microsoft repository and install the packages to your system using the
dpkg -i instruction.
- sudo dpkg -i packages-microsoft-prod.deb
Next, activate the Universe repository, which on Ubuntu is a community-maintained archive of software that is free and open source. This will give you access to
apt-transport-https, a dependency for enabling the Ubuntu package manager APT transport over HTTPS.
- sudo add-apt-repository universe
- sudo apt install apt-transport-https
Next, update available downloads:
- sudo apt update
Finally, install the current version of the .NET SDK. This tutorial will use version 2.2:
- sudo apt install dotnet-sdk-2.2
Now that you have the .NET SDK installed, a quick way to check if everything went well is to try the .NET Core command line interface (CLI), which will be available in the shell once the SDK is downloaded and installed. Display information about your .NET setup by typing this in your terminal:
- dotnet --info
When you run a
dotnet command for the first time, a text section is displayed as shown below:
OutputWelcome to .NET Core!
Learn more about .NET Core: https://aka.ms/dotnet-docs
Use 'dotnet --help' to see available commands or visit: https://aka.ms/dotnet-cli-docs
The .NET Core tools collect usage data in order to help us improve your experience. The data is anonymous and doesn't include command-line arguments. The data is collected by Microsoft and shared with the community. You can opt-out of telemetry by setting the DOTNET_CLI_TELEMETRY_OPTOUT environment variable to '1' or 'true' using your favorite shell.
Read more about .NET Core CLI Tools telemetry: https://aka.ms/dotnet-cli-telemetry
This notification is about collected data, and explains that some .NET CLI commands will send usage information to Microsoft. You will disable this in a moment; for now, look at the output from
After a brief moment, the terminal will list information about your .NET installation:
Output.NET Core SDK (reflecting any global.json):
OS Name: ubuntu
OS Version: 18.04
OS Platform: Linux
Base Path: /usr/share/dotnet/sdk/2.2.101/
Host (useful for support):
.NET Core SDKs installed:
.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.2.0 [/usr/share/dotnet/shared/Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.2.0 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.2.0 [/usr/share/dotnet/shared/Microsoft.NETCore.App]
To install additional .NET Core runtimes or SDKs:
Depending on the SDK version, the output may be slightly different, but this confirms that .NET Core is ready to use.
As mentioned before, the telemetry feature allows some .NET CLI commands to send usage information to Microsoft. It is enabled by default, and can be deactivated by setting the
DOTNET\_CLI\_TELEMETRY_OPTOUT environment variable to
1. To do so, add a new line to your
.profile environment customization file by opening it in your text editor. For this tutorial, we will use
- nano ~/.profile
Add the following line to the end of
. . .
nano by pressing the
X keys. When prompted to save the file, press
Y and then
You can activate the new configuration using the
- source ~/.profile
From now on, telemetry will be turned off at startup.
At this point you have .NET Core runtime, languages support, and libraries installed, allowing you to run and build some .NET applications. The
dotnet CLI is also available for managing .NET source code and binaries. You could start building F# projects, but as mentioned previously, the .NET Core environment does not provide all the constructs needed to be completely cross-platform. For now you cannot use it to develop mobile applications, for example.
In order to solve this problem, in the next step you will install F# again, but this time with Mono.
You can use Mono to fill in the remaining gaps in capability left by .NET Core. Mono and .NET Core are both based on the same standard library and both support .NET languages, but that is where the similarity ends. They use different runtimes, different CLIs, and different compilers, making it possible for them to be installed side by side to create a more reliable programming environment. In this section you are going to supplement your environment with the Mono tools for .NET programming and run an F# program from the command line.
A version of Mono is available in the Ubuntu repositories, but this can be outdated. Instead, add the official Mono package repository to your package manager:
- sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
- echo "deb https://download.mono-project.com/repo/ubuntu stable-bionic main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
In the preceding commands, you used
apt-key to retrieve keys for securing packages transferred from the official Mono repositories. You then added the Mono packages source to your repositories list.
With a new source list added for APT, update your repositories:
- sudo apt update
Next, download the Mono tools. Unlike .NET Core, Mono does not include F# tools, so you will download it as a separate package. Install
fsharp and the
mono-complete meta-package using the following command:
- sudo apt install mono-complete fsharp
Note: Because of the size of this download, the installation process for
mono-complete may take a while.
Once done, you will have the compiler
fsharpc and an interactive shell called
fsharpi or simply FSI. FSI is an environment, inside the shell, that receives user’s input as an expression, evaluates it, then outputs the result and waits for another input. It is just like typing a command in the traditional shell and seeing the result, except here, inputs are F# expressions. FSI provides a fast method to test code or run scripts.
Activate FSI with the following command:
This will start the interactive session and replace your regular prompt with the
OutputMicrosoft (R) F# Interactive version 4.1
Copyright (c) Microsoft Corporation. All Rights Reserved.
For help type #help;;
You can return to the default shell by running
fsharpi, each command line ends with a double semicolon.
Let’s try a simple operation using the
printfn function to render a message passed as a parameter:
- printfn "Hello World!";;
You will receive the following output:
val it : unit = ()
From the preceding interaction,
fsharpi evaluates the expression as a
unit type value. The code is then executed and the result is printed with its type.
fsharpi can also run a file containing F# code. The script must be named with a
.fsx extension and executed from the shell with the command:
- fsharpi some_script.fsx
Now that you know the F# installation is working, leave the shell with:
With Mono and .NET Core installed, you are now prepared to write any type of F# programs. FSI will allow you to test your code and run some scripts if needed, but executions will be slow. For your F# script to be executed, additional steps are performed to translate the source code into artifacts understandable by the processor, hence the slowness. To remedy this, in the next section you will compile your code with .NET Core, creating standalone binary files that can be immediately run by the machine.
In this step, you will compile F# source code via command line compilers provided with .NET Core. This will allow you to make your applications faster and to produce preset executable packages for specific systems, making your program easier to distribute.
Compiling is the transformation process that turns source code into binary file. The software that accomplishes this conversion is called a compiler. .NET Core relies on the
dotnet CLI to perform compiling. To demonstrate this, you are going to create a basic F# source to review the compilation cases.
dotnet CLI provides a complete application build toolchain. In general, an association of a command and the
dotnet driver is used in the shell to complete a task. For example:
dotnet new will create a project
dotnet build will build a project and all of its dependencies
dotnet add package will add a package reference to a project file
The following will create a new console project called
-lang option sets the programming language you will code with while the
-o option creates a directory in which to place the output.
- dotnet new console -lang F# -o FSharpHello
Once this is done, navigate into your newly created project directory:
- cd FSharpHello
This directory contains the
FSharpHello.fsproj project configuration file and the
obj folder which is used to store temporary object files. There is also the
Program.fs file where your default source code exists. Open it in your text editor:
- nano Program.fs
The file has been automatically filled with a Hello World program:
// Learn more about F# at http://fsharp.org
let main argv =
printfn "Hello World from F#!"
0 // return an integer exit code
In this code, you start importing the
System module with
open System, then you define the program entry point, i.e., the place where the program starts when launched from the shell. The
main function will call for a
Hello World message printing to the console and will stop the program (
return an integer exit code).
Exit out of the file.
To compile and run this code, use the following from the project directory
- dotnet run
The program will run, printing the following output to the screen:
OutputHello World from F#!
Note that it took a while for this program to run, just as with the FSI. As we mentioned before, it’s possible to run this faster by generating an executable, i.e., a binary file that can be run directly by the operating system. Here is how to achieve this:
- dotnet publish -c release -r linux-x64
This will produce the executable
bin/release/netcoreapp2.2/linux-x64/publish/FSharpHello.dll file. This is a shared library that will run on a 64-bit Linux architecture. To export a generic executable for macOS systems, you would replace the
linux-x64 runtime identifier (RID) with
Now execute the file with the following command:
- dotnet bin/release/netcoreapp2.2/linux-x64/publish/FSharpHello.dll
This time, you will receive the output much quicker, since the program is already translated into binary.
Now that you know how to compile in .NET Core, let’s see how Mono compiles programs with the dedicated
Mono’s compilation process is similar to that of .NET Core, but this time there is a specific command used to compile the program. The
fsharpc command is the tool, and it has been created only for compiling.
This time, create a
hello.fs file and write some F# code. First, return to your home directory:
Next, open up a new file named
- nano hello.fs
Add the following line to the file:
As seen before, this imports the
System module or namespace, giving you access to built-in system functions and objects like
Now, add in some more lines of code:
let hello() =
printf "Who are you? "
let name = Console.ReadLine()
printfn "Oh, Hello %s!\nI'm F#." name
These new lines define the
hello() function to read user input and print a feedback message.
Now you can add the final lines:
let hello() =
printf "Who are you? "
let name = Console.ReadLine()
printfn "Oh, Hello %s!\nI'm F#." name
Console.ReadKey() |> ignore
Here you are calling the function
hello(), then using the
ReadKey() method to end the program with a final keystroke.
Save and exit the file.
Now with the
fsharpc command, use the
-o flag to define the output filename and compile your
hello.fs source code like this:
- fsharpc hello.fs -o hello
The preceding command will generate a
hello executable file you can run with the
- mono hello
This gives you the following output and awaits user input:
OutputWho are you?
If you type in
Sammy, you will get the following.
OutputOh, Hello Sammy!
Press a final keystroke, and the program will end.
Congratulations! You have written and compiled your first F# program, both with Mono and .NET Core.
In this tutorial, you installed tooling for F# programming, covering both .NET Core and Mono environments. You also tested examples of F# code and built executables. These are the first steps toward learning this practical functional language.
Next steps could be to learn the language and get in touch with the community. Also, with projects getting more complex, you might need to manage code and resources more efficiently. Package managers like NuGet or Paket are bridges to the strong ecosystem built around .NET and tools-of-choice for organizing large programs.
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