Pascal is one of the landmark computer programming languages on which generations of students "cut their teeth" and variants of which are still widely used today. The popular typesetting system TeX and much of the original Macintosh operating system were written in Pascal.
The Swiss computer scientist Niklaus Wirth developed Pascal in 1970, first as a hypothetical language that would encourage students to write structured code. Pascal is based on the Algol programming language and is named in honor of mathematician and philosopher Blaise Pascal. Wirth also developed Modula-2 and Oberon, languages similar to Pascal which also support object-oriented programming.
Overview
All Pascal programs start with the "Program" keyword, an optional list of internal file descriptors and then a block of code is indicated with the "Begin" and "End" keywords. Letter case is ignored in the Pascal language. Semicolons separate statements, and the full stop ends the program (or unit). For some compilers the "Program" line is optional.
Pascal, in its original form, is a purely procedural language with the standard array of if, while, for, and related constructs.
Turbo Pascal, and other derivatives with units or module concepts are modular languages. Turbo Pascal copied these concepts from either a draft of the Extended Pascal standard or Pascal's successor Modula-2. However, it does not provide a nested module concept.
Hello World
program HelloWorld(input,output);
begin
Writeln('Hello, World!');
end.
Implementations
The first Pascal compiler was designed in Zurich for the CDC 6000 computer family, and it became operational in 1970.
The first Pascal compiler written in North America was constructed at the University of Illinois under Donald B. Gillies for the PDP-11 and generated native machine code
In order to rapidly propagate the language, a compiler "porting kit" was created in Zurich that included a compiler for a "virtual" machine code (or more properly an intermediate code), and a simulator for that same code. This became the P-system. Although this system was intended to enable true machine code compilers to be created, at least one system, the notable UCSD implementation, utilized it to create an interpretive system UCSD p-System.
In the 1980s Anders Hejlsberg wrote the Blue Label Pascal compiler for the Nascom-2. Later he went to work for Borland and rewrote his compiler to become Turbo Pascal for the IBM PC. This new compiler sold for $49.95, which was much less than the price Hejlsberg originally asked for the Blue Label Pascal compiler.
The inexpensive Borland compiler had a large influence on the Pascal community that began concentrating mainly on the IBM PC in the late 1980s. Many PC hobbyists in search of a structured replacement for BASIC used this product. Turbo Pascal, being available only on one architecture, translated directly to Intel 8088 machine code, making it much faster than interpreted schemes.
Super Pascal was a variant which added non-numeric labels, a return statement and expressions as names of types.
During the 1990s compilers that could be re-targeted to different hardware architectures became more prevalent. This allowed for Pascal translation to native machine code that was at the same time easily ported to new hardware.
With Turbo Pascal version 5.5 Borland added object orientation to Pascal.
However, Borland later decided it wanted more elaborate object-oriented features, and started over in Delphi using the Object Pascal draft standard proposed by Apple as a basis. (This Apple draft isn't a formal standard yet.) Borland also called this Object Pascal in the first Delphi versions, but changed the name to Delphi programming language in later versions. The main additions compared to the older OOP extensions were a reference-based object model, virtual constructors and destructors, and properties. There are several other compilers implementing this dialect: see Delphi programming language.
Standards
In 1983, the language was standardized, in the international standard ISO/IEC 7185, as well as several local country specific standards, including the American ANSI/IEEE770X3.97-1983. In 1990, an extended Pascal standard was created as ISO/IEC 10206.
The ISO 7185 was stated to be a clarification of Wirth's 1974 language as detailed by the User Manual and Report [Jensen and Wirth], but was also notable for adding "Conformant Array Parameters" as a level 1 to the standard, level 0 being Pascal without Conformant Arrays.
On the large machines (mainframes and minicomputers) Pascal originated on, the standards were generally followed. On the IBM-PC, they were not. On IBM-PCs, the Borland standards Turbo Pascal and Delphi have the greatest number of users. Thus, it is typically important to understand whether a particular implementation corresponds to the original Pascal language, or a Borland dialect of it.
Publicly available compilers
Several Pascal compilers are available for the use of general public:
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P4 compiler, the basis for many subsequent Pascal-implemented-in-Pascal compilers, including the UCSD p-System.
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Free Pascal is written in Pascal (so that it compiles itself), and is aimed at providing a convenient and powerful compiler, able both to compile legacy applications and to be the means of develop new ones. Also distributed freely under the GNU GPL. It can mix Turbo Pascal with Delphi code, and supports a lot of platforms and operating systems.
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Turbo Pascal was the dominant Pascal compiler for PCs during the 80s and early 90s, popular both because of its powerful extensions and extremely low compilation times. Currently, older versions of Turbo Pascal (up to 5.5) are available for free download from Borland's site (registration required).
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Chrome is a next generation Object Pascal language for the .NET and Mono Platforms implemented by RemObjects Software.
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GNU Pascal Compiler (GPC) is the Pascal compiler of the GNU Compiler Collection (GCC). The compiler itself is written in C, the runtime library mostly in Pascal. Distributed freely under the GNU General Public License, it runs on many platforms and operating systems. It supports the ANSI/ISO standard languages and the Borland/Turbo Pascal language largely. Support for Borland Delphi and other language variations is quite limited yet.
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Delphi is Borland's flagship RAD (Rapid Application Development) product. It uses the Delphi programming language, descended from Pascal, to create applications for the windows platform. The latest version also supports compiling to the .NET platform
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Kylix is Borland's newest reiteration of the Pascal branch of their products. It is the descendant of Delphi, with support for the Linux operating system and an improved object library. The compiler and the IDE are available now for non-commercial use. The compiler (but not the library or the IDE) is supposed to become Open Source software some time soon.
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Virtual Pascal was created by Vitaly Miryanov in 1995 as a native OS/2 compiler compatible with Borland Pascal syntax. Then it had been commercially developed by fPrint, adding Win32 support, and in 2000 it became freeware. Today it can compile for Win32, OS/2 and Linux, and is mostly compatible with Borland Pascal and Delphi.
A very extensive list can be found on Pascaland. The site is in French, but it is basically a list with URLs to compilers, so that doesn't matter.
Pascal and C
Pascal was developed about the same time as C, and there are important similarities between the two. Both were strongly influenced by Algol 60 and incorporate many of its essential features, including its block structure and type system, although the approach to type safety is quite different.
The original Pascal and straight C are both small procedural languages implementing structured programming concepts. Both have functionality for dynamic allocation of memory and some form of pointer manipulation. However, Pascal was designed, mainly, as a teaching language. Error-prone constructs were carefully avoided, and an effort was made to make the syntax easy to understand. The authors of C placed more emphasis on brevity.
The main differences between the original Pascal and C are the following:
- C has lots of operations which change their first operand (
++ -- += etc.)
- C has bitwise operations (
| & ^ ~ << >>), while Pascal has sets.
- C allows much greater freedom with pointers. In C, you can access any address in memory directly: given a variable a, an operation &a returns its address. There are functions
malloc() and calloc() which allocate any number of bytes. Pointers and arrays are often interchangeable. This makes C more concise than Pascal for low-level programming, but can lead to difficult to trace bugs.
- Pascal allows passing parameters either by value or by reference. In C, parameters can be passed only by value, and call by reference requires explicitly passing a pointer value.
- Pascal provides subrange types, which are very useful for early detection of bugs.
- Pascal lacks separate compilation .
- Pascal lacks preprocessing. There is nothing in original Pascal which resembles
#include, #define, #ifdef etc.
- In C, there is no boolean type.
- C lacks typesafe enumerated types which are present in Pascal.
- Pascal lacks
continue and break operators.
- Pascal permits nested function definitions.
- Pascal doesn't guarantee anything about certain border cases like if x is passed twice to the same procedure (once by reference, once by value). This is the classic, but true "why Pascal is faster than C" joke in NGs.
- the original Pascal disallows pointers inside of symbols. (e.g. @/& of a record field is disallowed), this has great consequences for implementation on top of a Virtual Machine
Past criticism
While very popular (although more so in the 1980s and early 1990s than at the time of writing), early versions of Pascal have been widely criticised for being unsuitable for "serious" use outside of teaching. Brian Kernighan, co-creator of the C programming language, outlined his most notable criticisms of Pascal as early as 1981, in his paper Why Pascal Is Not My Favorite Programming Language. On the other hand, many major development efforts in the 1980s, such as for the Apple Lisa and Macintosh, heavily depended on Pascal. In the decades since then, Pascal has continued to evolve and most of his points do not apply to current implementations. Unfortunately, most of the extensions to fix these issues are incompatible from compiler to compiler.
Further reading
See also
Last updated: 05-07-2005 09:50:09
Last updated: 05-13-2005 07:56:04
