Getting Started with F# in Visual Studio

Phillip Carter

F# and the Visual F# tooling are supported in the Visual Studio IDE. To begin, you should download Visual Studio, if you haven't already. This article uses the Visual Studio 2015 Community Edition, but you can use F# with the version of your choice.

Installing the Visual F# Tools

Visual Studio will first initialize the installer. After it is intilized, select Custom as shown here:

Select the Visual F# Tools under Programming Languages here:

Feel free to customize your installation further, and then continue with the installation. After a while, Visual Studio will complete installation and you can create an F# project!

Creating a Console Application

One of the most basic projects in Visual Studio is the Console Application. Here's how to do it. Once Visual Studio is open:

  1. On the File menu, point to New, and then choose Project.

  1. In the New Project dialog, under Templates, you should see Visual F#. Choose this to show the F# templates.

  1. Choose the Okay button to create the F# project! You should see something like this under Solution Explorer:

Writing your code

Let's get started by writing some code first. Make sure that the Program.fs file is open, and then replace its contents with the following:

module HelloSquare

let square x = x * x

let main argv =
    printfn "%d squared is: %d!" 12 (square 12)
    0 // Return an integer exit code

In the previous code sample, a function square has been defined which takes an input named x and multiplies it by itself. Because F# uses Type Inference, the type of x doesn't need to be specified. The F# compiler understands the types where multiplication is valid, and will assign a type to x based on how square is called. If you hover over square, you should see the following:

val square: x:int -> int

This is what is known as the function's type signature. It can be read like this: "Square is a function which takes an integer named x and produces an integer". Note that the compiler gave square the int type for now - this is because multiplication is not generic across all types, but rather is generic across a closed set of types. The F# compiler picked int at this point, but it will adjust the type signature if you call square with a different input type, such as a float.

Another function, main, is defined, which is decorated with the EntryPoint attribute to tell the F# compiler that program execution should start there. It follows the same convention as other C-style programming languages, where command-line arguments can be passed to this function, and an integer code is returned (typically 0).

It is in this function that we call the square function with an argument of 12. The F# compiler then assigns the type of square to be int -> int (that is, a function which takes an int and produces an int). The call to printfn is a formatted printing function which uses a format string, similar to C-style programming languages, parameters which correspond to those specified in the format string, and then prints the result and a new line.

Running Your Code

You can run the code and see results by pressing ctrl-f5. This will run the program without debugging and allows you to see the results. Alternatively, you can choose the Debug top-level menu item in Visual Studio and choose Start Without Debugging.

You should now see the following printed to the console window that Visual Studio popped up:

12 squared is 144!

Congratulations! You've created your first F# project in Visual Studio, written an F# function printed the results of calling that function, and run the project to see some results.

Using F# Interactive

One of the best features of the Visual F# tooling in Visual Studio is the F# Interactive Window. It allows you to send code over to a process where you can call that code and see the result interactively.

To begin using it, highlight the square function defined in your code. Next, hold the Alt key and press Enter. This executes the code in the F# Interactive Window. You should see the F# Interactive Window appear with the following in it:


val square : x:int -> int


This shows the same function signature for the square function, which you saw earlier when you hovered over the function. Because square is now defined in the F# Interactive process, you can call it with different values:

> square 12;;
val it : int = 144
>square 13;;
val it : int = 169

This executes the function, binds the result to a new name it, and displays the type and value of it. Note that you must terminate each line with ;;. This is how F# Interactive knows when your function call is finished. You can also define new functions in F# Interactive:

> let isOdd x = x % 2 <> 0;;

val isOdd : x:int -> bool

> isOdd 12;;
val it : bool = false

The above defines a new function, isOdd, which takes an int and checks to see if it's odd! You can call this function to see what it returns with different inputs. You can call functions within function calls:

> isOdd (square 15);;
val it : bool = true

You can also use the pipe-forward operator to pipeline the value into the two functions:

> 15 |> square |> isOdd;;
val it : bool = true

The pipe-forward operator, and more, are covered in later tutorials.

This is only a glimpse into what you can do with F# Interactive. To learn more, check out Interactive Programming with F#.

Next Steps

If you haven't already, check out the Tour of F#, which covers some of the core features of the F# language. It will give you an overview of some of the capabilities of F#, and provide ample code samples that you can copy into Visual Studio and run. There are also some great external resources you can use, showcased in the F# Guide.

See Also

Visual F#

Tour of F#

F# Language Reference

Type Inference

Symbol and Operator Reference