Wikipedia defines keyword
as:
In
computer programming, a keyword is a
word or
identifier that has a particular meaning to the
programming language. The meaning of keywords -- and, indeed,
the meaning of the notion of keyword -- differs widely from
language to language.
In many languages, such as
C and its kin, a keyword is a
reserved word which identifies a syntactic form. Words used in
control flow constructs, such as
if,
then, and
else are
keywords. In these languages, keywords cannot also be used as the
names of variables or functions.
Some languages, such as
PostScript, are extremely liberal in this approach (such as
PostScript), allowing core keywords to be redefined for specific
purposes.
In
Common Lisp, in contrast, keywords (or keyword symbols)
are a special sort of symbol, or identifier. Unlike other symbols,
which usually stand for variables or functions, keywords are self-quoting
and evaluate to themselves. Keywords are usually used to label named
arguments to functions, and to represent symbolic values.
Languages vary as to what is provided as a keyword and what is a
library routine. Some languages, for instance, provide keywords
for input/output operations whereas in others these are library
routines. In
Python and many
BASIC
dialects, print
is a keyword. In contrast, the C and Lisp equivalents
printf and
format
are functions in the standard library.
In languages with macros or lazy evaluation, control flow
constructs such as if
can be implemented as macros or functions. In languages without
these expressive features, they are generally keywords.
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