Pascal (programming language)
Pascal is an imperative and procedural programming language, designed by Niklaus Wirth as a small, efficient language intended to encourage good programming practices using structured programming and data structuring. It is named after French mathematician, philosopher and physicist Blaise Pascal.
For other uses, see Pascal (disambiguation).
Pascal was developed on the pattern of the ALGOL 60 language. Wirth was involved in the process to improve the language as part of the ALGOL X efforts and proposed a version named ALGOL W. This was not accepted, and the ALGOL X process bogged down. In 1968, Wirth decided to abandon the ALGOL X process and further improve ALGOL W, releasing this as Pascal in 1970.
On top of ALGOL's scalars and arrays, Pascal enables defining complex datatypes and building dynamic and recursive data structures such as lists, trees and graphs. Pascal has strong typing on all objects, which means that one type of data cannot be converted to or interpreted as another without explicit conversions. Unlike C (and most languages in the C-family), Pascal allows nested procedure definitions to any level of depth, and also allows most kinds of definitions and declarations inside subroutines (procedures and functions). A program is thus syntactically similar to a single procedure or function. This is similar to the block structure of ALGOL 60, but restricted from arbitrary block statements to just procedures and functions.
Pascal became very successful in the 1970s, notably on the burgeoning minicomputer market. Compilers were also available for many microcomputers as the field emerged in the late 1970s. It was widely used as a teaching language in university-level programming courses in the 1980s, and also used in production settings for writing commercial software during the same period. It was displaced by the C programming language during the late 1980s and early 1990s as UNIX-based systems became popular, and especially with the release of C++.
A derivative named Object Pascal designed for object-oriented programming was developed in 1985. This was used by Apple Computer (for the Lisa and Macintosh machines) and Borland in the late 1980s and later developed into Delphi on the Microsoft Windows platform. Extensions to the Pascal concepts led to the languages Modula-2 and Oberon, both developed by Wirth.
History[edit]
Earlier efforts[edit]
Much of the history of computer language design during the 1960s can be traced to the ALGOL 60 language. ALGOL was developed during the 1950s with the explicit goal of being able to clearly describe algorithms. It included a number of features for structured programming that remain common in languages to this day.
Shortly after its introduction, in 1962 Wirth began working on his dissertation with Helmut Weber on the Euler programming language. Euler was based on ALGOL's syntax and many concepts but was not a derivative. Its primary goal was to add dynamic lists and types, allowing it to be used in roles similar to Lisp. The language was published in 1965.
By this time, a number of problems in ALGOL had been identified, notably the lack of a standardized string system. The group tasked with maintaining the language had begun the ALGOL X process to identify improvements, calling for submissions. Wirth and Tony Hoare submitted a conservative set of modifications to add strings and clean up some of the syntax. These were considered too minor to be worth using as the new standard ALGOL, so Wirth wrote a compiler for the language, which became named ALGOL W.
The ALGOL X efforts would go on to choose a much more complex language, ALGOL 68. The complexity of this language led to considerable difficulty producing high-performance compilers, and it was not widely used in the industry. This left an opening for newer languages.
Pascal[edit]
Pascal was influenced by the ALGOL W efforts, with the explicit goals of teaching programming in a structured fashion and for the development of system software.[4]
A generation of students used Pascal as an introductory language in undergraduate courses.
One of the early successes for the language was the introduction of UCSD Pascal, a version that ran on a custom operating system that could be ported to different platforms. A key platform was the Apple II, where it saw widespread use as Apple Pascal. This led to Pascal becoming the primary high-level language used for development in the Apple Lisa, and later, the Macintosh. Parts of the original Macintosh operating system were hand-translated into Motorola 68000 assembly language from the Pascal source code.[5]
The typesetting system TeX by Donald Knuth was written in WEB, the original literate programming system, based on DEC PDP-10 Pascal. Successful commercial applications like Adobe Photoshop[6] were written in Macintosh Programmer's Workshop Pascal, while applications like Total Commander, Skype[7] and Macromedia Captivate were written in Delphi (Object Pascal). Apollo Computer used Pascal as the systems programming language for its operating systems beginning in 1980.
Variants of Pascal have also been used for everything from research projects to PC games and embedded systems. Newer Pascal compilers exist which are widely used.[8]
Dialects[edit]
Wirth's example compiler meant to propagate the language, the Pascal-P system, used a subset of the language designed to be the minimal subset of the language that could compile itself. The idea was that this could allow bootstrapping the compiler, which would then be extended to full Pascal language status. This was done with several compilers, but one notable exception was UCSD Pascal, which was based on Pascal-P2. It kept the subset status of the language based on the idea that this would run better on the new (then) microprocessors with limited memory. UCSD also converted the Pascal-P2 interpreter into a "byte machine", again, because it would be a better fit for byte oriented microprocessors.
UCSD Pascal formed the basis of many systems, including Apple Pascal. Borland Pascal was not based on the UCSD codebase, but arrived during the popular period of UCSD and matched many of its features. This started the line that ended with Delphi Pascal and the compatible Open Source compiler FPC/Lazarus.
The ISO standard for Pascal, ISO 7185, was published in 1983 and was widely implemented and used on mainframes, minicomputers and IBM-PCs and compatibles from 16 bits to 32 bits. The two dialects of Pascal most in use towards the end of the 20th century and up until today are the ISO 7185 standard version and the Delphi/Turbo Pascal versions (of which the two Borland versions are mostly compatible with each other).
The source for much of the early history on Pascal can be found in the Pascal User's Group newsletters at:
Pascal Users Group Newsletters.
Object Pascal[edit]
During work on the Lisa, Larry Tesler began corresponding with Wirth on the idea of adding object-oriented extensions to the language, to make Pascal a Multi-paradigm programming language. This led initially to Clascal, introduced in 1983. As the Lisa program faded and was replaced by the Macintosh, a further version was created and named Object Pascal. This was introduced on the Mac in 1985 as part of the MacApp application framework, and became Apple's main development language into the early 1990s.
The Object Pascal extensions were added to Turbo Pascal with the release of version 5.5 in 1989.[9] Over the years, Object Pascal became the basis of the Delphi system for Microsoft Windows, which is still used for developing Windows applications, and can cross-compile code to other systems. Free Pascal is an open source, cross-platform alternative with its own graphical IDE called Lazarus.
Implementations[edit]
Early Pascal compilers[edit]
The first Pascal compiler was designed in Zürich for the CDC 6000 series mainframe computer family. Niklaus Wirth reports that a first attempt to implement it in FORTRAN 66 in 1969 was unsuccessful due to FORTRAN 66's inadequacy to express complex data structures. The second attempt was implemented in a C-like language (Scallop by Max Engeli) and then translated by hand (by R. Schild) to Pascal itself for boot-strapping.[10] It was operational by mid-1970. Many Pascal compilers since have been similarly self-hosting, that is, the compiler is itself written in Pascal, and the compiler is usually capable of recompiling itself when new features are added to the language, or when the compiler is to be ported to a new environment. The GNU Pascal compiler is one notable exception, being written in C.
The first successful port of the CDC Pascal compiler to another mainframe was completed by Welsh and Quinn at the Queen's University of Belfast (QUB) in 1972. The target was the International Computers Limited (ICL) 1900 series. This compiler, in turn, was the parent of the Pascal compiler for the Information Computer Systems (ICS) Multum minicomputer. The Multum port was developed – with a view to using Pascal as a systems programming language – by Findlay, Cupples, Cavouras and Davis, working at the Department of Computing Science in Glasgow University. It is thought that Multum Pascal, which was completed in the summer of 1973, may have been the first 16-bit implementation.
A completely new compiler was completed by Welsh et al. at QUB in 1977. It offered a source-language diagnostic feature (incorporating profiling, tracing and type-aware formatted postmortem dumps) that was implemented by Findlay and Watt at Glasgow University. This implementation was ported in 1980 to the ICL 2900 series by a team based at Southampton University and Glasgow University. The Standard Pascal Model Implementation was also based on this compiler, having been adapted, by Welsh and Hay at Manchester University in 1984, to check rigorously for conformity to the BSI 6192/ISO 7185 Standard and to generate code for a portable abstract machine.
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.
The Pascal-P system[edit]
To propagate the language rapidly, a compiler porting kit was created in Zürich that included a compiler that generated so called p-code for a virtual stack machine, i.e., code that lends itself to reasonably efficient interpretation, along with an interpreter for that code – the Pascal-P system. The P-system compilers were named Pascal-P1, Pascal-P2, Pascal-P3, and Pascal-P4. Pascal-P1 was the first version, and Pascal-P4 was the last to come from Zürich. The version termed Pascal-P1 was coined after the fact for the many different sources for Pascal-P that existed. The compiler was redesigned to enhance portability, and issued as Pascal-P2. This code was later enhanced to become Pascal-P3, with an intermediate code backward compatible with Pascal-P2, and Pascal-P4, which was not backward compatible.
The Pascal-P4 compiler–interpreter can still be run and compiled on systems compatible with original Pascal (as can Pascal-P2). However, it only accepts a subset of the Pascal language.
Pascal-P5, created outside the Zürich group, accepts the full Pascal language and includes ISO 7185 compatibility.
Pascal-P6 is a follow on to Pascal-P5 that along with other features, aims to be a compiler for specific CPUs, including AMD64.
UCSD Pascal branched off Pascal-P2, where Kenneth Bowles used it to create the interpretive UCSD p-System. It was one of three operating systems available at the launch of the original IBM Personal Computer.[11] UCSD Pascal used an intermediate code based on byte values, and thus was one of the earliest bytecode compilers. Apple Pascal was released in 1979 for the Apple II and Apple III computer systems. It was an implementation of, or largely based on, UCSD Pascal. Pascal-P1 through Pascal-P4 were not, but rather based on the CDC 6600 60-bit word length.
A compiler based on the Pascal-P4 compiler, which created native binary object files, was released for the IBM System/370 mainframe computer by the Australian Atomic Energy Commission; it was named the AAEC Pascal 8000 Compiler after the abbreviation of the name of the commission.[12]
Object Pascal and Turbo Pascal[edit]
Apple Computer created its own Lisa Pascal for the Lisa Workshop in 1982, and ported the compiler to the Apple Macintosh and MPW in 1985. In 1985 Larry Tesler, in consultation with Niklaus Wirth, defined Object Pascal and these extensions were incorporated in both the Lisa Pascal and Mac Pascal compilers.
In the 1980s, Anders Hejlsberg wrote the Blue Label Pascal compiler for the Nascom-2. A reimplementation of this compiler for the IBM PC was marketed under the names Compas Pascal and PolyPascal before it was acquired by Borland and renamed Turbo Pascal.
Turbo Pascal became hugely popular, thanks to an aggressive pricing strategy, having one of the first full-screen IDEs, and very fast turnaround time (just seconds to compile, link, and run). It was written and highly optimized entirely in assembly language, making it smaller and faster than much of the competition.
In 1986, Anders ported Turbo Pascal to the Macintosh and incorporated Apple's Object Pascal extensions into Turbo Pascal. These extensions were then added back into the PC version of Turbo Pascal for version 5.5. At the same time Microsoft also implemented the Object Pascal compiler.[13][14] Turbo Pascal 5.5 had a large influence on the Pascal community, which 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. It also began to be adopted by professional developers. Around the same time a number of concepts were imported from C to let Pascal programmers use the C-based application programming interface (API) of Microsoft Windows directly. These extensions included null-terminated strings, pointer arithmetic, function pointers, an address-of operator, and unsafe typecasts.
Turbo Pascal and other derivatives with unit or module structures are modular programming languages. However, it does not provide a nested module concept or qualified import and export of specific symbols.
Other variants[edit]
Super Pascal adds non-numeric labels, a return statement and expressions as names of types.
TMT Pascal was the first Borland-compatible compiler for 32-bit MS-DOS compatible protected mode, OS/2, and Win32. It extends the language with function and operator overloading.
The universities of Wisconsin–Madison, Zürich, Karlsruhe, and Wuppertal developed the Pascal-SC[15][16] and Pascal-XSC[17][18][19] (Extensions for Scientific Computation) compilers, aimed at programming numerical computations. Development for Pascal-SC started in 1978 supporting ISO 7185 Pascal level 0, but level 2 support was added at a later stage.[20] Pascal-SC originally targeted the Z80 processor, but was later rewritten for DOS (x86) and 68000. Pascal-XSC has at various times been ported to Unix (Linux, SunOS, HP-UX, AIX) and Microsoft/IBM (DOS with EMX, OS/2, Windows) operating systems. It operates by generating intermediate C source code which is then compiled to a native executable. Some of the Pascal-SC language extensions have been adopted by GNU Pascal.
Pascal Sol was designed around 1983 by a French team to implement a Unix-like system named Sol. It was standard Pascal level-1 (with parameterized array bounds) but the definition allowed alternative keywords and predefined identifiers in French and the language included a few extensions to ease system programming (e.g. an equivalent to lseek).[21] The Sol team later on moved to the ChorusOS project to design a distributed operating system.[22]
IP Pascal is an implementation of the Pascal programming language using Micropolis DOS, but was moved rapidly to CP/M-80 running on the Z80. It was moved to the 80386 machine types in 1994, and exists today as Windows XP and Linux implementations. In 2008, the system was brought up to a new level and the resulting language termed "Pascaline" (after Pascal's calculator). It includes objects, namespace controls, dynamic arrays, and many other extensions, and generally features the same functionality and type protection as C#. It is the only such implementation that is also compatible with the original Pascal implementation, which is standardized as ISO 7185.
Standards[edit]
ISO/IEC 7185:1990 Pascal[edit]
In 1983, the language was standardized in the international standard IEC/ISO 7185[33] and several local country-specific standards, including the American ANSI/IEEE770X3.97-1983, and ISO 7185:1983. These two standards differed only in that the ISO standard included a "level 1" extension for conformant arrays (an array where the boundaries of the array are not known until run time), where ANSI did not allow for this extension to the original (Wirth version) language. In 1989, ISO 7185 was revised (ISO 7185:1990) to correct various errors and ambiguities found in the original document.
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. This addition was made at the request of C. A. R. Hoare, and with the approval of Niklaus Wirth. The precipitating cause was that Hoare wanted to create a Pascal version of the (NAG) Numerical Algorithms Library, which had originally been written in FORTRAN, and found that it was not possible to do so without an extension that would allow array parameters of varying size. Similar considerations motivated the inclusion in ISO 7185 of the facility to specify the parameter types of procedural and functional parameters.
Niklaus Wirth himself referred to the 1974 language as "the Standard", for example, to differentiate it from the machine specific features of the CDC 6000 compiler. This language was documented in The Pascal Report,[34] the second part of the "Pascal users manual and report".
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.
The IBM PC versions of the language began to differ with the advent of UCSD Pascal, an interpreted implementation that featured several extensions to the language, along with several omissions and changes. Many UCSD language features survive today, including in Borland's dialect.