Your First F# Program

The traditional way to start an F# project is dotnet new console -lang "F#" -o HelloFSharp, which scaffolds a minimal console application containing a single file, Program.fs, along with a .fsproj project file that tells the .NET build system which source files to compile and in what order. Unlike many languages, F# compiles files top-to-bottom and depends on their order in the .fsproj, so understanding that file matters even for a simple 'Hello, World!' program.

Inside Program.fs, the generated template defines a main function decorated with the [<EntryPoint>] attribute, which tells the compiler this is where program execution begins — much like Main in C# or main in C. The function must accept a string array of command-line arguments and return an int exit code (0 conventionally means success); inside it, printfn "Hello, World!" writes text to the console, where printfn is F#'s type-safe formatted-print function, using placeholders like %s for strings, %d for integers, and %f for floats, all checked by the compiler at compile time.

You can run the program two ways: dotnet run from the project folder compiles the whole project and executes the resulting binary, which is the standard workflow for a real application; alternatively, for quick experiments you can skip the project entirely and write a .fsx script file, then execute it directly with dotnet fsi hello.fsx, which is faster for throwaway exploration since there's no project file or build step involved, just the F# Interactive engine interpreting the script.

A slightly richer first program demonstrates more of F#'s character than a bare 'Hello, World!': defining a small helper function above main, calling it with a list of names, and printing a formatted greeting for each one shows function definition, list iteration with List.iter, and printfn's multiple format specifiers working together in a single small file — a realistic taste of how F# code is typically structured even in much larger programs.