| Paradigm(s) | imperative, structured |
|---|---|
| Appeared in | 1970 |
| Designed by | Niklaus Wirth |
| Typing discipline | static, strong, safe |
| Major implementations | CDC 6000, ICL 1900, Pascal-P, PDP-11, PDP-10, IBM System/370, HP, GNU |
| Dialects | Borland, Turbo Pascal, UCSD Pascal |
| Influenced by | ALGOL W |
| Influenced | Ada, Component Pascal, Java,[1][2][3] Modula, Modula-2, Modula-3, Oberon, Oberon-2, Object Pascal, Oxygene |
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.
A derivative known as Object Pascal designed for object-oriented programming was developed in 1985.
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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).
Object Pascal is still used for developing Windows applications such as Skype. A cross-platform version called Free Pascal, with the Lazarus IDE, is popular with Linux users since it promises write once, compile anywhere, development.
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.)
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.
The universities of Zurich, Karlsruhe and Wuppertal have developed an EXtension for Scientific Computing (Pascal XSC), which provides a free solution for programming numerical computations with controlled precision.
Pascal, in its original form, is a purely procedural language and includes the traditional array of ALGOL-like control structures with reserved words such as if, then, else, while, for, and so on. However, Pascal also has many data structuring facilities and other abstractions which were not included in the original ALGOL 60, like type definitions, records, pointers, enumerations, and sets. Such constructs were in part inherited or inspired from Simula 67, ALGOL 68, Niklaus Wirth's own ALGOL W and suggestions by C. A. R. Hoare.
Pascal programs start with the program keyword with a list of external file descriptors as parameters;[8] then follows the main block bracketed by the begin and end keywords. Semicolons separate statements, and the full stop (i.e., a period) ends the whole program (or unit). Letter case is ignored in Pascal source.
Here is an example of the source code in use for a very simple "Hello world" program:
program HelloWorld(output); begin Writeln('Hello world!') end.
A type in Pascal, and in several other popular programming languages, defines a variable in such a way that it defines a range of values which the variable is capable of storing, and it also defines a set of operations that are permissible to be performed on variables of that type. The predefined types are:
| Data type | Type of values which the variable is capable of storing |
|---|---|
| integer | Whole numbers |
| real | Floating point numbers |
| boolean | The value TRUE or FALSE |
| char | A single character from an ordered character set |
The range of values allowed for each (except boolean) is implementation defined. Functions are provided for some data conversions. For conversion of real to integer, the following functions are available: round, which round to integer using banker's rounding; trunc, round towards zero.
The programmer has the freedom to define other commonly-used data types (e.g. byte, string, etc.) in terms of the predefined types using Pascal's type declaration facility. e.g.
type byte = 0..255; signedbyte = -128..127; string = packed array [1..255] of char;
Subranges of any ordinal data type (any simple type except real) can be made:
var x: 1..10; y: 'a'..'z'; z: pear..orange;
In contrast with other programming languages from its time, Pascal supports a set type:
var set1: set of 1..10; set2: set of 'a'..'z'; set3: set of pear..orange;
A set is a fundamental concept for modern mathematics, and they may be used in many algorithms. Such a feature is useful and may be faster than an equivalent construct in a language that does not support sets. For example, for many Pascal compilers:
if i in [5..10] then ...
executes faster than:
if (i>4) and (i<11) then ...
Sets of non-contiguous values can be particularly useful, in terms of both performance and readability:
if i in [0..3, 7, 9, 12..15] then ...
For these examples, which involve sets over small domains, the improved performance is usually achieved by the compiler representing set variables as bitmasks. The set operators can then be implemented efficiently as bitwise machine code operations.
Types can be defined from other types using type declarations:
type x = Integer; y = x; ...
Further, complex types can be constructed from simple types:
type a = Array [1..10] of Integer; b = record x: Integer; y: Char end; c = File of a;
As shown in the example above, Pascal files are sequences of components. Every file has a buffer variable which is denoted by f^. The procedures get (for reading) and put (for writing) move the buffer variable to the next element. Read is introduced such that read(f, x) is the same as x:=f^; get(f);. Write is introduced such that write(f, x) is the same as f^ := x; put(f); The type text is predefined as file of char. While the buffer variable could be used for inspecting the next character to be used (check for a digit before reading an integer), this leads to serious problems with interactive programs in early implementations, but was solved later with the "lazy I/O" concept.
In Jensen & Wirth Pascal, strings are represented as packed arrays of chars; they therefore have fixed length and are usually space-padded. Some dialects have a custom string type.
Pascal supports the use of pointers:
type Nodeptr = ^Node; Node = record a: Integer; b: Char; c: Nodeptr end; var ptoNode: Nodeptr; pInt : ^Integer;
Here the variable ptoNode is a pointer to the data type Node, a record. Pointers can be used before they are declared. This is a forward declaration, an exception to the rule that things must be declared before they are used. To create a new record and assign the value 10 and character A to the fields a and b in the record, and to initialise the pointer c to nil, the commands would be:
new(ptoNode); ... ptoNode^.a := 10; ptoNode^.b := 'A'; ptoNode^.c := nil; ...
This could also be done using the with statement, as follows
new(ptoNode); ... with ptoNode^ do begin a := 10; b := 'A'; c := nil end; ...
Inside of the scope of the with statement, a and b refer to the subfields of the record pointer ptoNode and not to the record Node or the pointer type Nodeptr.
Linked lists, stacks and queues can be created by including a pointer type field (c) in the record (see also nil).
Unlike many languages that feature pointers, Pascal only allows pointers to reference dynamically created variables that are anonymous, and does not allow them to reference standard static or local variables. Pointers also must have an associated type, and a pointer to one type is not compatible with a pointer to another type (e.g. a pointer to a char is not compatible with a pointer to an integer). This helps eliminate the type security issues inherent with other pointer implementations, particularly those used for PL/I or C. It also removes some risks caused by dangling pointers, but the ability to dynamically let go of referenced space by using the dispose function (which has the same effect as the free library function found in C) means that the risk of dangling pointers has not been entirely eliminated.[9]
Pascal is a structured programming language, meaning that the flow of control is structured into standard statements, ideally without 'goto' commands.
while a <> b do writeln('Waiting'); if a > b then writeln('Condition met') else writeln('Condition not met'); for i := 1 to 10 do writeln('Iteration: ', i:1); repeat a := a + 1 until a = 10; case i of 0: write('zero'); 1: write('one'); 2: write('two') end;
Pascal structures programs into procedures and functions.
program mine(output); var i : integer; procedure print(var j: integer);
Procedures and functions can nest to any depth, and the 'program' construct is the logical outermost block.
Each procedure or function can have its own declarations of goto labels, constants, types, variables, and other procedures and functions, which must all be in that order. This ordering requirement was originally intended to allow efficient single-pass compilation. However, in some dialects (such as Embarcadero Delphi) the strict ordering requirement of declaration sections has been relaxed.
Pascal adopted many language syntax features from the ALGOL language, including the use of a semicolon as a statement separator. This is in contrast to other languages, such as PL/I, C etc. which use the semicolon as a statement terminator. As illustrated in the above examples, no semicolon is needed before the end keyword of a record type declaration, a block, or a case statement; before the until keyword of a repeat statement; and before the else keyword of an if statement.
The presence of an extra semicolon was not permitted in early versions of Pascal. However, the addition of ALGOL-like empty statements in the 1973 Revised Report and later changes to the language in ISO 7185:1983 now allow for optional semicolons in most of these cases. The exception is that a semicolon is still not permitted immediately before the else keyword in an if statement.
Several Pascal compilers and interpreters are available for the use of general public:
A very extensive list can be found on Pascaland. The site is in French, but it is basically a list with URLs to compilers; there is little barrier for non-Francophones. The site, Pascal Central, a Mac centric Pascal info and advocacy site with a rich collection of article archives, plus links to many compilers and tutorials, may also be of interest.
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.
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.
Pascal generated a wide variety of responses in the computing community, both critical and complimentary.
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).
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.
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, Oberon and Oberon-2. 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.
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