Pascal is an influential
imperative and
procedural programming language, designed in 1968/9 and published in 1970 by
Niklaus Wirth as a small and efficient language intended to encourage good programming practices using
structured programming and
data structuring.
History
Pascal was developed by
Niklaus Wirth and based on the
ALGOL programming language, named in honor of the French mathematician and philosopher
Blaise Pascal.
Prior to his work on Pascal, Wirth had developed
Euler and
ALGOL W and later went on to develop the Pascal-like languages
Modula-2 and
Oberon.
Initially, Pascal was largely, but not exclusively, intended to teach students
structured programming.
[4] A generation of students used Pascal as an introductory language in undergraduate courses. Variants of Pascal have also frequently been used for everything from research projects to
PC games and
embedded systems. Newer Pascal compilers exist which are widely used.
[5]
Pascal was the primary high-level language used for development in the
Apple Lisa, and in the early years of the Mac. Parts of the original
Macintosh operating system were hand-translated into
Motorola 68000 assembly language from the Pascal sources. The popular
typesetting system
TeX by
Donald E. Knuth was written in
WEB, the original
literate programming system, based on
DEC PDP-10 Pascal, while an application like
Total Commander was written in Delphi (
Object Pascal).
Brief description
Wirth's intention was to create an efficient language (regarding both compilation speed and generated code) based on so-called
structured programming, a concept which had recently become popular. Pascal has its roots in the
ALGOL 60 language, but also introduced concepts and mechanisms which (on top of ALGOL's scalars and arrays) enabled programmers to define their own complex (structured) datatypes, and also made it easier to build dynamic and recursive data structures such as
lists,
trees and
graphs. Important features included for this were
records,
enumerations,
subranges,
dynamically allocated variables with associated
pointers, and
sets. To make this possible and meaningful, Pascal has a
strong typing on all objects, which means that one type of data cannot be converted or interpreted as another without
explicit conversions. Similar mechanisms are standard in many programming languages today. Other languages that influenced Pascal's development were
COBOL,
Simula 67, and Wirth's own
ALGOL W.
Pascal, like many programming languages of today (but unlike most languages in the C family), allows nested procedure definitions to any level of depth, and also allows most kinds of definitions and declarations inside procedures and functions. This enables a very simple and coherent syntax where a complete
program is syntactically nearly identical to a single
procedure or
function (except for the keyword itself, of course.)
[edit] Implementations
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 in 1969 was unsuccessful due to Fortran's inadequacy to express complex data structures. The second attempt was formulated in the Pascal language itself and 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
QUB in 1972. The target was the
ICL 1900 series. This compiler in turn was the parent of the Pascal compiler for the
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.
To propagate the language rapidly, a compiler "porting kit" was created in Zurich that included a compiler that generated 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 termed Pascal-P1, Pascal-P2, Pascal-P3, and Pascal-P4. Pascal-P1 was the first version, and Pascal-P4 was the last to come from Zurich.
The Pascal-P4 compiler/interpreter can still be run and compiled on systems compatible with original Pascal. However, it only accepts a subset of the Pascal language.
Pascal-P5, created outside of the Zurich group, accepts the full Pascal language and includes ISO 7185 compatibility.
UCSD Pascal branched off Pascal-P2, where
Kenneth Bowles utilized it to create the
interpretive UCSD p-System
A compiler based on the Pascal-P4 compiler, which created native binaries, was released for the
IBM System/370 mainframe computer by the
Australian Atomic Energy Commission; it was called the "AAEC Pascal Compiler" after the abbreviation of the name of the Commission.
In the early 1980s,
Watcom Pascal was developed, also for the IBM
System 370.
IP Pascal was an implementation of the Pascal programming language using Micropolis DOS, but was moved rapidly to
CP/M 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, along with many other extensions, and generally features the same functionality and type protection as C#. It is the only such implementation which is also compatible with the original Pascal implementation (which is standardized as ISO 7185).
In the early 1980s,
UCSD Pascal was ported to the
Apple II and
Apple III computers to provide a structured alternative to the
BASIC interpreters that came with the machines.
Apple Computer created its own Lisa Pascal for the Lisa Workshop in 1982 and ported this 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. Renamed to
Turbo Pascal it became hugely popular, thanks in part to an aggressive pricing strategy and in part to having one of the first full-screen
Integrated development environments, and fast turnaround-time (just seconds to compile, link, and run.) Additionally, 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.
[6][7] 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
API of
Microsoft Windows directly. These extensions included null-terminated
strings,
pointer arithmetic,
function pointers, an address-of operator and unsafe
typecasts.
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 is still not a formal standard.) The first versions of the Delphi language were accordingly named Object Pascal. The main additions compared to the older OOP extensions were a reference-based object model, virtual constructors and destructors, and properties. Several other compilers also implement this dialect.
Turbo Pascal, and other derivatives with units or module concepts are
modular languages. However, it does not provide a nested module concept or qualified import and export of specific symbols.
Super Pascal was a variant which added non-numeric labels, a return statement and expressions as names of types.
Standards
In 1983, the language was standardized, in the international standard IEC/ISO 7185, 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, 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.
In 1990, an extended Pascal standard was created as ISO/IEC 10206. In 1993 the ANSI standard was replaced by the ANSI organization with a "pointer" to the ISO 7185:1990 standard, effectively ending its status as a different standard.
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.
Note that 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", 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.
[edit] Divisions
Niklaus Wirth's Zurich version of Pascal was issued outside of
ETH in two basic forms, the CDC 6000 compiler source, and a porting kit called Pascal-P system. The Pascal-P compiler left out several features of the full language. For example, procedures and functions used as parameters, undiscriminated variant records, packing, dispose, interprocedural gotos and other features of the full compiler were omitted.
UCSD Pascal, under Professor
Kenneth Bowles, was based on the Pascal-P2 kit, and consequently shared several of the Pascal-P language restrictions. UCSD Pascal was later adopted as Apple Pascal, and continued through several versions there. Although UCSD Pascal actually expanded the subset Pascal in the Pascal-P kit by adding back standard Pascal constructs, it was still not a complete standard installation of Pascal.
Borland's
Turbo Pascal, written by
Anders Hejlsberg, was written in assembly language independent of UCSD or the Zurich compilers. However, it adopted much of the same subset and extensions as the UCSD compiler. This is probably because the UCSD system was the most common Pascal system suitable for developing applications on the resource-limited microprocessor systems available at that time.
[edit] List of related standards
- ISO 8651-2:1988 Information processing systems—Computer graphics—Graphical Kernel System (GKS) language bindings—Part 2: Pascal
[edit] Reception
Pascal generated a wide variety of responses in the computing community, both critical and complimentary.
[edit] Criticism
While very popular (although more so in the 1980s and early 1990s than now), implementations of Pascal which closely followed Wirth's initial definition of the language were widely criticized for being unsuitable for use outside of teaching.
Brian Kernighan, who popularized the
C language, outlined his most notable criticisms of Pascal as early as 1981, in his paper
Why Pascal Is Not My Favorite Programming Language.
[10] The most serious problem, described in this article, seems that array sizes and string lengths were part of the type so it was not possible to write a function that would accept variable length arrays or even strings as parameters (like a sorting library, for instance). The author also criticized the unpredictable order of evaluation of boolean expressions, poor library support, lack of static variables and a number of smaller issues. Also, he stated that the language did not provide any simple constructs to "escape" (knowingly and forcibly ignore) restrictions and limitations where this is really necessary. (However, there is a feature of "record variants" that does allow such an "escape," though it is decidedly cumbersome.) More general complaints from other sources
[11][12] noted that the scope of declarations were not clearly defined in the original language definition, which sometimes had serious consequences when using
forward declarations to define pointer types, or when record declarations lead to
mutual recursion, or when an identifier may or may not have been used in an enumeration list. Another difficulty was that, like
ALGOL 60, the language did not allow procedures or functions passed as parameters to pre-define what their parameters are supposed to be.
On the other hand, many major development efforts in the 1980s, such as for the
Apple Lisa and
Macintosh, heavily depended on Pascal (to the point where the C interface for the Macintosh operating system API had to deal in Pascal data types).
[edit] Reactions
Pascal continued to evolve, and most of Kernighan's points do not apply to versions of the language which were enhanced to be suitable for commercial product development, such as Borland's
Turbo Pascal. Unfortunately, just as Kernighan predicted in his article, most of the extensions to fix these issues were incompatible from compiler to compiler. Since the early 1990s, however, the varieties seem to have condensed into two categories, ISO and Borland-like, a better eventual outcome than Kernighan foresaw.
[original research?]
Although Kernighan decried Pascal's lack of type escapes ("there is no escape" from "Why Pascal is not my Favorite Programming language"), the uncontrolled use of
pointers and type escapes have become highly criticized features in their own right, and the languages
Java,
C# and others feature a sharp turn-around to the Pascal point of view. What these languages call "managed pointers" were in fact foreseen by Wirth with the creation of Pascal.
Based on his experience with Pascal (and earlier with ALGOL)
Niklaus Wirth developed several more programming languages:
Modula,
Modula-2 and
Oberon. These languages address some criticisms of Pascal, are intended for different user populations, and so on, but none has had the widespread impact on computer science and computer users as has Pascal, nor has any yet met with similar commercial success
Some of them feel incomplete without wearing high heels, among them need to add height (four to five inches or more), few feel confident and sexy when walking in high heels, and usually very easy to be tempted on the shoes with elegant design. It was like an "addiction".
Third, avoid pain. To avoid abrasions and blisters due to glue the plaster on the sensitive part of your foot, especially the back of the heel and toe sections. Use a tape that matches the color stockings, if not, use transparent tape as a substitute. Chinese proverb say bite the back of the shoes before you wear them, so the shoes will not bite you. Rest whenever you can. Rest your feet on the seat or at the bus stop. Try to reduce standing or walking, forbid feel tired in the muscles.
The females asked ranged in their opinions from positive to negative. Some use gels and sprays each day and love them, and some prefer not using any 
As a rule they say that some salon products are more expensive, but worth the results.
Some of the females felt that coloring on the whole was not a good idea; that it was not good for the hair, plus all the upkeep and expense involved to keep the look. 
Because of the expense, they felt that doing it at home with a kit was preferable to going to a salon. They liked the idea of highlights, but not bleaching.
Well, this brought out a flurry of opinions! Females as a rule preferred 
It came as no surprise that females enjoy experimenting with their hair more than males. Males agreed that once you find a look that works, keep it! 
Now for the big question--what styles do females love to see males wear? What styles do males like on females?