Having provided a basic introduction to F#, a discussion over its primitive types, and an in-depth look at units of measure I thought now would be a good time to take a step back and look at one of the helpful tools used when developing F# applications – F# Interactive, or FSI. FSI is a REPL-like (read-evaluate-print loop) utility that’s especially useful for experimenting with the language.

FSI is considered REPL-like because although it fills the same role as a traditional REPL tool it actually compiles the code rather than interpreting it. This distinction is important because it impacts the tool’s behavior. Types and values are commonly redefined in REPL tools but because FSI is compiling the code into new assemblies it only offers an illusion of redefinition. Everything already defined is still available and instances defined against previous definitions aren’t affected as long as they were defined in the same session.  FSI does enforce that if a type changes any new instances are created against the new definition but it’s important to be aware that those changes will not be reflected in any instances created before the change.

FSI is available as both a window within Visual Studio or as a console application. If you’re actively developing an application in Visual Studio you’ll probably find the F# Interactive window more helpful because you can select snippets of code and send them to the window for execution by pressing ALT + ENTER. The console version is especially well suited for running F# scripts (.fsx files).

You can open the F# Interactive window in Visual Studio by pressing CTRL + ALT + F or through the View/Other Windows menu.

Whether running under Visual Studio or the console, operation is the same. Expressions are entered at the prompt and terminated with double semicolons (;;). FSI then attempts compilation and, if successful, prints the result of the expression evaluation.

For every new name introduced by the input, the FSI output will include a val entry. Anything that returns a value but does not have a name are represented as “it.”

You can easily reset an FSI session in Visual Studio by right-clicking the window and selecting the Reset Session option.

Common FSI Directives

FSI includes some directives that aren’t available with the compiler. A few of these are especially useful for scripting.

More Reading

• F# Interactive (FSI) Reference
• F# Interactive Options