This chapter presents an overview of the syntax and primary constructs of the Visual Basic.NET (VB.NET) language for programmers unfamiliar with VB.NET. This is not a tutorial chapter, however, so if you are new to programming you should study another text on VB.NET before continuing with this book. If, though, you are coming to VB.NET from some other language, such as C++ or Java or even Visual Basic 6, you should read through this chapter to familiarize yourself with the language.

NET PROGRAMS

There are two ways to build programs in VB.NET. One is to use the Visual Studio.NET Integrated Development Environment (IDE). The other is to use the command-line compiler packaged as part of the.NET Framework Software Development Kit (SDK). In this section we'll discuss developing programs with the command-line compiler, since this software is free and can run on any of the modern Windows operating sysems (Windows 98 and beyond).

VB.NET Program Types

With VB.NET, you can write many different kinds of programs. A VB.NET program that makes use of a graphical user interface (GUI) is a Windows application. A VB.NET program that uses the command-prompt console for input and output is called a Console application. You can also write Internet applications, Windows Services applications, and other types of applications.

A Windows application is by necessity written in an object-oriented manner. This chapter discusses the code generated by VS. NET when a Windows application is built. We'll examine both the code generated by VS. NET and the code created when a programmer builds an application. We will also examine how to use object-oriented principles in the code the programmer adds to the application.

As the noted computer scientist and education theorist Seymour Paper once stated, “You can't think about thinking without thinking about thinking about something.” Likewise, you can't examine building a Windows application without having a Windows application to build. To that end, the application we'll use to illustrate how object-oriented principles aid the creation of a Windows application is a basic calculator, similar to the one found in Windows. By picking a rather simple application that uses many widely employed Windows features (e.g., textboxes, buttons, and labels), we can focus on how taking an object-oriented approach makes the development easier and more efficient. We should also mention that this chapter is not designed to teach a novice how to create a Windows application; rather it teaches how to create a Windows application in an object-oriented way. We assume the reader already knows how to place controls on a form and already knows the basics of writing event-driven code.

Of all modern programming languages, Standard ML has ascribed perhaps the highest priority to rigorous semantic definition. It is therefore the preferred language for many applications where rigor is important; this is notably true of tools for formal program analysis. It has also gained users who value its high degree of portability, a direct consequence of the unambiguity of its definition.

Now Emden Gansner and John Reppy have equipped SML with another essential ingredient: a library of signatures, structures, and functors which will greatly ease the programmer's task. The SML Basis Library has been long in gestation, but this has ensured that it contains the right things. Only by close cooperation with users, over a considerable period of time, can one be sure of consistency and balance in defining a library. We can therefore be confident that the Basis Library will bring SML into still wider use, and we owe warm thanks to its creators for undertaking an arduous task with skill, care, and dedication.

One essential for the success of a general-purpose language is an accompanying standard library that is rich enough and efficient enough to support the basic, day-to-day tasks common to all programming. Libraries provide the vocabulary with which a language can be used to say something about something. Without a broad common vocabulary, a language community cannot prosper as it might.

This document presents a standard basis library for SML. It is a basis library in the sense that it concerns itself with the fundamentals: primitive types such as integers and floating-point numbers, operations requiring runtime system or compiler support, such as I/O and arrays; and ubiquitous utility types such as booleans and lists. The SML Basis Library purposefully does not cover higher-level types, such as collection types, or application-oriented APIs, such as regular expression matching. The primary reason for limiting the scope in this way is that the design space for these interfaces is large (e.g. choosing between functors and polymorphism as a parameterization mechanism) and, unlike the case with lists and arrays, we do not have many years of common practice to guide the design. It is also the case that the SML Basis Library specification is a substantial document and expanding its scope would make it unwieldy.

The primary purpose of this book is to serve as a reference manual for the Basis Library, describing as clearly and completely as possible the types, values, and modules making up the Library.

Overview

Sockets are an abstraction for interprocess communication (IPC) that were introduced as part of the Berkeley version of Unix in 1982. They have become a de facto standard for network communication and are supported by most major operating systems (including PC systems). The SML Basis Library provides an optional collection of modules for programming with sockets. The interface provided by the Basis follows the C interface for the most part; the major difference is that the SML interface is more strongly typed. In particular, the type system distinguishes between passive and active sockets, between sockets in different domains, and between sockets of different protocols.

The Berkeley Socket API supports two styles of communication: stream sockets provide virtual circuits between pairs of processes, and datagram sockets provide connectionless packet-based communication. In stream-based interactions, the server allocates a master socket that is used to accept connections from clients. The server then listens on the master socket for connection requests from clients; each request is allocated a new socket that the server uses to communicate with that particular client. As the name suggests, stream-based communication is done as a stream of bytes, not as discrete packets. Connectionless communication is more symmetric: messages are sent to a specific port at a specific address. While datagram sockets provide better performance, messages may be lost or received out of order, which requires additional programming by the client. For this reason, stream sockets are more commonly used than datagram sockets.