Your First Program

Create a file named hello.fsx:

printfn "Hello, world!"
printfn "Welcome to F#."

Run it:

dotnet fsi hello.fsx

Expected output

Hello, world!
Welcome to F#.

• printfn prints text followed by a new line; printf prints without the new line.
• Strings go in double quotes.
• No semicolons, no braces, no main function: a script runs from top to bottom.

Format specifiers

printfn takes a format string with placeholders, and checks the arguments' types at compile time:

let name = "Ada"
let age = 36

printfn "%s is %d years old" name age
printfn "Pi is roughly %.2f" 3.14159
printfn "Anything: %A" [1; 2; 3]
printfn $"Interpolated: {name} is {age}"

Ada is 36 years old
Pi is roughly 3.14
Anything: [1; 2; 3]
Interpolated: Ada is 36

%s is a string, %d an integer, %f a float (%.2f with two decimals), and %A pretty-prints any value. The last line uses an interpolated string — $"..." with expressions in braces.

A console project

For a real application, create a project:

dotnet new console -lang F# -o HelloApp
cd HelloApp
dotnet run

The template's Program.fs contains a single line, printfn "Hello from F#", which is what dotnet run prints.

Practice

Write a script that prints your name on one line and, on the next line, the sentence "I am learning F#" using a %s placeholder for the word "F#".

printfn "Ada"
printfn "I am learning %s" "F#"

Ada
I am learning F#