F# Data Types Overview

Tuples

The first example demonstrates constructing and deconstructing a tuple type. Tuple is simple type that groups together two or more values of any (possibly different) types, for example int and string:

>let tuple = (42, "Hello world!");; val tuple : int * string > let (num, str) = tuple;; val num : int val str : string

As you can see, the compiler deduced a type of the expression that is present on the right side of the equals sign and the F# interactive printed the type, so we can review it. In this example the type of a first element in a tuple is int and the type of the second element is string. The asterisk denotes that the type is a tuple. Similarly, you can define a tuple with more than three elements, but the type changes with the number of elements in a tuple, which means that tuples can't be used for storing an unknown number of values. This can be done using lists or arrays, which will be discussed later.

The syntax used for deconstructing the value into variables num and str is in general called pattern matching and it is used very often in the F# language – the aim of pattern matching is to allow matching a value against a pattern that specifies different view of the data type – in case of tuple, one view is a single value (of type tuple) and the second view is a pair of two values (of different types). Pattern matching can be used with all standard F# types, most notably with tuples, discriminated unions and record types. In addition, F# also supports generalized pattern matching constructs called active patterns, which are discussed later in this overview.

Tuple types are very handy for returning multiple values from functions, because this removes the need to declare a new class or use references when writing a function that performs some simple operation resulting in more returned values (especially in places where C# uses ref and out parameters). In general, I would recommend using tuples when the function is either simple (like division with remainder), local (meaning that it will not be accessed from a different module or file) or it is likely to be used with pattern matching. For returning more complicated structures it is better to use record types which will be discussed shortly.

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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.

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Introducing F#

In one of the papers about F#, the F# designers gave the following description: "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 (don’t worry if you don’t know these languages, we’ll look at some examples shortly), 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.

Finally, the third paradigm supported by F# is language oriented programming (the design of F# in this area is largely influenced by ML, Haskell and also by LINQ). In general, language oriented programming is focused on developing executors for some code which has a structure of a language (be it a declarative language like XML, or a fully powerful language like some subset of F#). In this overview, I will focus on two techniques provided by F# that allow you to give a different meaning to blocks of F# code. In a programming language theory, this is often called internal domain specific languages, because the code is written in the host language, but is specifically designed as a way for solving problems from some specific domain. An example of such language (and an associated executor) is a block of code that is written as a linear code, but is executed asynchronously (in F# this can be implemented using computation expressions), or a query that is written in F#, but is executed as a SQL code by some database server (this can be implemented using F# quotations).

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Introduction

This text is based on a short overview of the F# language that was included in my bachelor thesis, but I extended it to cover all the important F# aspects and topics. The goal of this article is to introduce all the features in a single (relatively short) text, which means that understanding of a few advanced topics discussed later in the text may require some additional knowledge or previous experience with functional programming.

Anyway, the article still tries to introduce all the interesting views on programming that the F# language provides and the goal is to show that these views are interesting, even though not all of them are fully explained as they would deserve. Of course, this text won’t teach you everything about F#, but it tries to cover the main F# design goals and (hopefully) presents all
the features that make F# interesting and worth learning. In this first part I will shortly introduce F# and the supported paradigms that will be discussed further in the text.

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