F# is a general-purpose, statically typed language for building robust, succinct, and performant applications on .NET. Its lightweight syntax, type inference, and compiler support let code focus on domain problems rather than repeated type annotations, ceremony, or class scaffolding. It runs across Windows, macOS, and Linux, and interoperates with C# so existing .NET libraries and hybrid solutions remain available. section 1.1 section 2.2
The central working model is data plus functions. Data represents application state; small functions transform one data type into another; and composition connects those transformations into pipelines. This favors reuse through functions rather than inheritance, while F# can still use classes, interfaces, properties, and methods when they suit a .NET integration. section 1.5.5 section 2.1.2 section 2.2
Types are practical domain-modeling tools, not merely annotations. Cheap, specific types can encode business rules and prevent accidental mixing of similar data, such as a city and country that would otherwise both be strings. Compiler feedback then exposes missing assignments, unhandled cases, and optional values while code is being changed, shifting work from runtime failures toward explicit design decisions. section 1.5.2 section 1.5.4 section 2.1.4
F# supports a broad range of application work:
- Create command-line tools, background services, and desktop applications. section 1.3
- Build cloud services, web services, and web applications with .NET and ASP.NET Core. section 1.3
- Target browsers through Fable, or Android and iOS through Xamarin or MAUI. section 1.5.7
- Explore data or sketch components interactively before moving successful code into an application. section 1.5.1 section 2.7
Compose focused transformations
A typical F# design starts with a small operation, such as selecting even numbers, and combines it with another operation, such as squaring numbers. The composed function expresses the larger behavior without introducing a coordinating class, keeping each transformation independently understandable. section 2.1.2
IEnumerable<int> SquareNumbers(IEnumerable<int> numbers) {
// implementation of square elided...
}
IEnumerable<int> GetEvenNumbersThenSquare(IEnumerable<int> numbers) {
return SquareNumbers(GetEvenNumbers(numbers)) #1
}
let squareNumbers = Seq.map (fun x -> x * x)
let getEvenNumbersThenSquare = getEvenNumbers >> squareNumbers #2
Compare nested method calls with an F# composition operator that connects the same two transformations directly. section 2.1.2
Interactive scripts and the REPL make experimentation part of normal development. Model a problem with in-memory data, test an idea cheaply, then migrate the result into a console application, service, or web API. Keeping business logic separate from databases and web services also makes focused testing and ad hoc exploration easier. section 1.5.1 section 1.5.6 section 2.7
