Pascal-S

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 in honour of the French mathematician, philosopher and physicist Blaise Pascal.

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.

History

Earlier efforts

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

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.
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.

The typesetting system TeX by Donald E. Knuth was written in WEB, the original literate programming system, based on DEC PDP-10 Pascal. Successful commercial applications like Adobe Photoshop were written in Macintosh Programmer's Workshop Pascal, while applications like Total Commander, Skype 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.

Object Pascal

During work on the Lisa, Larry Tesler began corresponding with Wirth on the idea of adding object-oriented extensions to the 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. 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

Early Pascal compilers

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. 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

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. 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.

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. 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 2 and Apple 3 computer systems. It was an implementation of, or largely based on, UCSD Pascal. Pascal-P1 through Pascal-P4 was not, but rather based on the CDC 6600 60-bit word length.

A compiler based on the Pascal-P5 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.

Object Pascal and Turbo Pascal

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. 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

Super Pascal is a variant that added non-numeric labels, a return statement and expressions as names of types.

TMT Pascal was the first Borland-compatible compiler for 32-bit DOS protected mode, OS/2, and Win32 operating systems. The TMT Pascal language was the first one to allow function and operator overloading.

The universities of Wisconsin-Madison, Zürich, Karlsruhe, and Wuppertal developed the ''Pascal-SC'' and ''Pascal-XSC'' ('' 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. 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). The Sol team later on moved to the ChorusOS project to design a distributed operating system.

IP Pascal was 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.

Language constructs

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 case, ranging on a single statement or a begin-end statements block. Pascal also has data structuring constructs not included in the original ALGOL 60 types, like records, variants, pointers, enumerations, and sets and procedure pointers. 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 parametersPascal ISO 7185:1990
6.10 (not required in Turbo Pascal etc.); 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
Write('Hello, World!')

end.

Data types

A Type Declaration in Pascal is used to define a range of values which a variable of that type is capable of storing. It also defines a set of operations that are permissible to be performed on variables of that type. The predefined types are:

The range of values allowed for the basic types (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 rounds to integer using banker's rounding) and trunc (rounds 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, for example

:
type
byte = 0..255;
signed_byte = -128..127;
string = packed array ..255of char;

Often-used types like byte and string are already defined in many implementations.

Normally the system will use a word to store the data. For instance, the type may be stored in a machine integer - 32 bits perhaps - rather than an 8-bit value. Pascal does not contain language elements that allow the basic storage types to be defined more granularly. This capability was included in a number of Pascal extensions and follow-on languages, while others, like Modula-2, expanded the built-in set to cover most machine data types like 16-bit integers.

The keyword tells the compiler to use the most efficient method of storage for the structured data types: sets, arrays and records, rather than using one word for each element. Packing may slow access on machines that do not offer easy access to parts of a word.

Subrange types

Subranges of any ordinal data type (any simple type except real) can also be made:

:
var
x : 1..10;
y : 'a'..'z';

Set types

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';

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 ..10then ...

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 ..3, 7, 9, 12..15then ...

For these examples, which involve sets over small domains, the improved performance is usually achieved by the compiler representing set variables as bit vectors. The set operators can then be implemented efficiently as bitwise machine code operations.

Union types

Type declarations

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 ..10of integer;
b = record
x : integer;
y : char
end;
c = file of a;

File type

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 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.

Pointer types

Pascal supports the use of pointers:

:
type
pNode = ^Node;
Node = record
a : integer;
b : char;
c : pNode
end;
var
NodePtr : pNode;
IntPtr : ^integer;

Here the variable ''NodePtr'' 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 the null pointer ("NIL" in Pascal), the statements would be:

:
New(NodePtr);
...
NodePtr^.a := 10;
NodePtr^.b := 'A';
NodePtr^.c := NIL;
...

This could also be done using the with statement, as follows:

:
New(NodePtr);
...
with NodePtr^ 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 ''NodePtr'' and not to the record Node or the pointer type pNode.

Linked lists, stacks and queues can be created by including a pointer type field (c) in the record.

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 deallocate 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 eliminatedJ. Welsh, W. J. Sneeringer, and C. A. R. Hoare, "Ambiguities and Insecurities in Pascal", ''Software: Practice and Experience 7'', pp. 685–696 (1977) as it has in languages such as Java and C#, which provide automatic garbage collection (but which do not eliminate the related problem of memory leaks).

Some of these restrictions can be lifted in newer dialects.

Control structures

Pascal is a structured programming language, meaning that the flow of control is structured into standard statements, usually 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);

repeat
a := a + 1
until a = 10;

case i of
0 : Write('zero');
1 : Write('one');
2 : Write('two');
3,4,5,6,7,8,9,10: Write('?')
end;

Procedures and functions

Pascal structures programs into procedures and functions. Generally, a procedure is used for its side effects, whereas a function is used for its return value.

:
program Printing;

var i : integer;

procedure PrintAnInteger(j : integer);
begin
...
end;

function triple(const x: integer): integer;
begin
triple := x * 3
end;

begin
...
PrintAnInteger(i);
PrintAnInteger(triple(i))
end.

Procedures and functions can be nested to any depth, and the 'program' construct is the logical outermost block.

By default, parameters are passed by value. If 'var' precedes a parameter's name, it is passed by reference.

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 Delphi) the strict ordering requirement of declaration sections has been relaxed.

Semicolons as statement separators

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 and C, which use the semicolon as a statement terminator. 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. A semicolon is still not permitted immediately before the else keyword in an ''if'' statement, because the else follows a single statement, not a statement sequence. In the case of nested ifs, a semicolon cannot be used to avoid the dangling else problem (where the inner if does not have an else, but the outer if does) by putatively terminating the nested if with a semicolon – this instead terminates both if clauses. Instead, an explicit begin...end block must be used.

Resources

Compilers and interpreters

Several Pascal compilers and interpreters are available for general use:

* Delphi is Embarcadero's (formerly Borland/CodeGear) flagship rapid application development (RAD) product. It uses the Object Pascal language (termed 'Delphi' by Borland), descended from Pascal, to create applications for Windows, macOS, iOS, and Android. The .NET support that existed from D8 through D2005, D2006, and D2007 has been terminated, and replaced by a new language (Prism, which is rebranded Oxygene, see below) that is not fully backward compatible. In recent years Unicode support and generics were added (D2009, D2010, Delphi XE).
* Free Pascal is a cross-platform compiler written in Object Pascal (and is self-hosting). It is aimed at providing a convenient and powerful compiler, both able to compile legacy applications and to be the means to develop new ones. It is distributed under the GNU General Public License (GNU GPL), while packages and runtime library come under a modified GNU Lesser General Public License (GNU LGPL). In addition to compatibility modes for Turbo Pascal, Delphi, and Mac Pascal, it has its own procedural and object-oriented syntax modes with support for extended features such as operator overloading. It supports many platforms and operating systems. Current versions also feature an ISO mode.
* Turbo51 is a free Pascal compiler for the Intel 8051 family of microcontrollers, with Turbo Pascal 7 syntax.
* Oxygene (formerly named ''Chrome'') is an Object Pascal compiler for the .NET and Mono platforms. It was created and is sold by RemObjects Software, and sold for a while by Embarcadero as the backend compiler of Prism.
* Kylix was a descendant of Delphi, with support for the Linux operating system and an improved object library. It is no longer supported. Compiler and IDE are available now for non-commercial use.
* GNU Pascal Compiler (GPC) is the Pascal compiler of the GNU Compiler Collection (GCC). The compiler is written in C, the runtime library mostly in Pascal. Distributed under the GNU General Public License, it runs on many platforms and operating systems. It supports the ANSI/ISO standard languages and has partial Turbo Pascal dialect support. One of the more painful omissions is the absence of a 100% Turbo Pascal-compatible (short)string type. Support for Borland Delphi and other language variants is quite limited. There is some support for Mac-pascal, however.
* 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. Development was canceled on April 4, 2005.
* P4 compiler, the basis for many subsequent Pascal-implemented-in-Pascal compilers. It implements a subset of full Pascal.
* P5 compiler is an ISO 7185 (full Pascal) adaption of P4.
* Smart Mobile Studio is a Pascal to HTML5/Javascript compiler
* Turbo Pascal was the dominant Pascal compiler for PCs during the 1980s and early 1990s, popular both because of its powerful extensions and extremely short compilation times. Turbo Pascal was compactly written and could compile, run, and debug all from memory without accessing disk. Slow floppy disk drives were common for programmers at the time, further magnifying Turbo Pascal's speed advantage. Currently, older versions of Turbo Pascal (up to 5.5) are available for free download from Borland's site.
* IP Pascal implements the language "Pascaline" (named after Pascal's calculator), which is a highly extended Pascal compatible with original Pascal according to ISO 7185. It features modules with namespace control, including parallel tasking modules with semaphores, objects, dynamic arrays of any dimensions that are allocated at runtime, overloads, overrides, and many other extensions. IP Pascal has a built-in portability library that is custom tailored to the Pascal language. For example, a standard text output application from 1970's original Pascal can be recompiled to work in a window and even have graphical constructs added.
* Pascal-XT was created by Siemens for their mainframe operating systems BS2000 and SINIX

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