“F# is a multi-paradigm .NET language explicitly designed to be an ML suited to the .NET environment. It is rooted in the Core ML design and in particular has a core language largely compatible with OCaml”. In other words this means that the syntax of the F# language is similar to ML or OCaml but the F# language targets .NET Framework, which means that it can natively work with other .NET components and also that it contains several language extensions to allow smooth integration with the .NET object system.
Another important aspect mentioned in this description is that F# is multi-paradigm language. This means that it tries to take the best from many programming languages from very different worlds. The first paradigm is functional programming (the languages that largely influenced the design of F# in this area are ML, OCaml and other), which has a very long tradition and is becoming more important lately for some very appealing properties, including the fact that functional code tends to be easier to test and parallelize and is also extensible in a ways where object oriented code makes extending difficult.
The second paradigm is widely adopted object oriented programming, which enables interoperability with other .NET languages. In F# it is often used for implementing elementary data types (meaning that the operations on the type are well known and change very rarely), for grouping a set of elementary functions that are together used to perform some complicated operation (i.e. implementing an interface) and also when working with object oriented user interface frameworks.
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, 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.
Similarly as ML and OCaml, F# adopts an eager evaluation mechanism, which means that a code written using sequencing operator is executed in the same order in which it is written and expressions given as an arguments to a function are evaluated before calling the function (this mechanism is used in most imperative languages including C#, Java or Python). This makes it semantically reasonable to support imperative programming features in a functional language. As already mentioned, the F# value bindings are by default immutable, so to make a variable mutable the mutable keyword has to be used. Additionally F# supports a few imperative language constructs (like for and while), which are expressions of type unit:
Object oriented constructs in F# are compatible with the OO support in .NET CLR, which implies that F# supports single implementation inheritance (a class can have one base class), multiple interface inheritance (a class can implement several interfaces and an interface can inherit from multiple interfaces), subtyping (an inherited class can be casted to the base class type) and dynamic type tests (it is possible to test whether a value is a value of an inherited class casted to a base type). Finally, all object types share a common base class which is called obj in F# and is an alias to the CLR type System.Object.
Defining precisely what the term language oriented programming means in context of the F# language would be difficult, In general, the goal of language oriented programming is to develop a language that would be suitable for some (more specific) class of tasks and use this language for solving these tasks. Of course, developing a real programming language is extremely complex problem, so there are several ways for making it easier. As the most elementary example, you can look at XML files (with certain schema) as language that are processed by your program and solve some specific problem (for example configuring the application). The main reason why language oriented development is appealing paradigm is that it allows very smooth cooperation of people working on the project – there are people who develop the language and those who use it. The language developers need to have advanced knowledge of the technology (F#) and also of the problem that their language is trying to solve (e.g. some mathematical processing), but they don’t need to know about all the problems that are being solved using the language. On the other side, the users of the language need only basic F# knowledge and they can fully focus on solving the real problems.
F# combines the simplicity and elegance of typed functional programming with the strengths of the .NET platform. Although typed functional programming is relatively new to many programmers and thus requires some learning, it makes programming simpler in many ways. F# programs tend to be built from compositional, correct foundational elements, and type inference makes programs shorter and clearer. In the article, i first introduces the foundational paradigms of F#: functional programming, imperative programming, and object-oriented programming, and emphasizes how these can be used together. Focus is on simple, clear explanations of the foundational elements of the language, with an eye on the enjoyment that comes from programming in general, and programming with F# in particular.