The only difference between a "library" and a "full application" is that the "full application" has an entry point where execution begins, whereas a "library" d

The only difference between a "library" and a "full application" is that the "full application" has an entry point where execution begins, whereas a "library" does not. Many large applications are implemented as a thin user interface (GUI, web, services/APIs) which glues together a bunch of libraries (which is where all of the real functionality is implemented). This doesn't only apply to F#, but C#, Java, Python, etc. The reason F# gets used for implementing libraries is because that makes it easier to introduce into organizations with existing code in C# or VB.NET; a new component or plugin can be written in F# and easily utilized by the existing code, so the other developers in the organization don't need to anything differently than if they were consuming another C# or VB.NET component.