Functional Programming

As already mentioned, F# is a typed functional language, by which I mean that types of all values are determined during the compile-time. However, thanks to the use of a type inference, the types are explicitly specified in the code very rarely as we will see in the following examples. The type inference means that the compiler deduces the type from the code, so for example when calling a function that takes int as an argument and returns string as a result, the compiler can infer the type of the variable where the result is assigned (it has to be string) as well as the type of the variable that is given as an argument (it has to be int). Basic data types (aside from a standard set of primitive numeric and textual types that are present in any .NET language) available in F# are tuple, discriminated union, record, array, list, function and object. In the following quick overview, we will use the F# interactive, which is a tool that compiles and executes the entered text on the fly.
The F# interactive can be either used from Visual Studio or by running the fsi.exe from the F# installation directory. In the whole article series we will also use the F# lightweight syntax, which makes the code white-space sensitive, but simplifies many of the syntactical rules. To enable the lightweight syntax enter the following command in the FSI:

> #light;;

The double semicolon is used as the end of the FSI input and sends the entered text to the compiler. This allows us to enter multiple lines of code in a single command. With a few exceptions (mostly when showing a declaration of a complex type) all the code samples in this article are written as commands for FSI including the double semicolon and the result printed by the FSI. Longer code samples can be entered to FSI as well - just add the double semicolon to terminate the input.