.NET Type System—Relationship Between Programming Languages and Type Systems
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The Relationship Between Programming Languages and Type Systems
The Evolution of Type Systems
Why is a type system necessary at all? Some early programming languages did not provide a type system; they simply saw memory as a sequence of bytes. This perspective required developers to manually craft their own "types" to represent user-defined abstractions. For example, if a developer needed four bytes to represent integer values, then he or she had to write code to allocate four bytes for these integers and then manually check for overflow when adding two integers, byte by byte.
Later programming languages provided type systems, which included a number of built-in abstractions for common programming types. The first type systems were very low level, providing abstractions for fundamental types, such as characters, integers, and floating-point numbers, but little more. These types were commonly supported by specific machine instructions that could manipulate them. As type systems become more expressive and powerful, programming languages emerged that allowed users to define their own types.
Of course, type systems provide more benefits than just abstraction. Types are a specification, which the compiler uses to validate programs through a mechanism such as static type checking. (In recent years, dynamic type checking has become more popular.) Types also serve as documentation, allowing developers to more easily decipher code and understand its intended semantics. Unfortunately, the type systems provided by many programming languages are incompatible, so language integration requires the integration of different types to succeed.Programming Language-Specific Type Systems
Before attempting to design a type system for use by multiple languages, let's briefly review the type systems used by some of the more popular programming languages.
The C programming language provides a number of primitive built-in types, such as int and float. These types are said to closely resemble a machine's architecture, as they can often be held in a single register and may have specific machine instructions to process them. The C programmer can also create user-defined types, such as enumerations or structures. Structures are essentially aggregate types that contain members of one or more other types.
The C++ programming language takes the type system of C and extends it with object-oriented and generic programming facilities. C++'s classes (essentially C structures) can inherit from multiple other classes and extend these classes' functionality. C++ does not provide any new built-in types but does offer libraries, such as the Standard Template Library (STL), that greatly enhance the language's functionality.
SmallTalk is an object-oriented language in which all types are classes. SmallTalk's type system provides single-implementation inheritance, and every type usually directly or indirectly inherits from a common base class called Object,1 providing a common root class in the SmallTalk type system. SmallTalk is an example of a dynamically type-checked language.
Like SmallTalk, Java provides an object-oriented type system; unlike SmallTalk, it also supports a limited number of primitive built-in types. Java provides a single-implementation inheritance model with multiple inheritance of interfaces.
 
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