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).
Object Pascal is still widely used for developing Windows applications such as
Skype.
[citation needed]
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.)
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.
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.
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 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.
Hello world
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 (ie, 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.
Data types
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;
Scalar types
Pascal's scalar types are
real,
integer,
character,
boolean and
enumerations, a new type constructor introduced with Pascal:
type
SomeType = (State1,State2,State3);
var
r: Real;
i: Integer;
c: Char;
b: Boolean;
t: SomeType;
e: (apple, pear, banana, orange, lemon);
Subrange types
Subranges of any ordinal type (any simple type except real) can be made:
var
x: 1..10;
y: 'a'..'z';
z: pear..orange;
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';
set3: set of pear..orange;
A set is a fundamental concept for modern mathematics, and they may be used in a 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:
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.
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 [1..10] of 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 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.
Pointer types
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]
Control structures
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;
Procedures and functions
Pascal structures programs into procedures and functions.
program mine(output);
var i : integer;
procedure print(var j: integer);
function next(k: integer): integer;
begin
next := k + 1
end;
begin
writeln('The total is: ', j);
j := next(j)
end;
begin
i := 1;
while i <= 10 do print(i)
end.
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.
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,
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.
