TOPS-20 PASCAL Language Manual - Bitsavers

INTRODUCTION
1.2.3 Identifiers
PASCAL uses the term identifier to mean the name of a program, module,
constant, type, variable, procedure, or function. An identifier is a
sequence of characters that CRn include letters, digits, dollar signs
( $ ), and un de r 1 i n e s ym b 0 1 s ( ), wit h the foIl ow i ng res t ric t ion s :
• An identifier can begin with any character other than a
dig it.
• An identifier must be unique in its first 31 characters
within the block in which it is declared.
• An identifier must not contain any blanks.
PASCAL places no restrictions on the length of identifiers, but scans
only the first 31 characters for uniqueness; the rest are ignored.
~he following are examples of valid and invalid identifiers:
Va 1 id
Fo r2n8
Max Words
Upto
$CREMBX
Invalid
4awhile (starts with a digit)
Up&to (contains the ampersand)
1.2.3.1 Predeclared Identifiers - PASCAL predeclares some identifiers
as names of functions, procedures, types, values, and files. These
predeclared identifiers are listed in Table 1-2 and appear in
uppercase characters throughout this manual.
Table 1-2:
Predeclared Identifiers
------
ABS EXP OPEN SINGLE
ARCTAN EXPO ORD SNGL
BOOLEAN FALSE OUTPUT SQR
CARD FIND PACK SQRT
CHAR GET PAGE SUCC
CHR HALT PRED TEXT
CLOCK INPU1r PUT TIME
CLOSE INTEGER READ TRUE
COS LINELIMIT READLN TRUNC
DATE LN REAL UNDEFINED
DISPOSE MAXINT RESET UNPACK
DOUBLE NEW REWRITE WRITE
EOF ODD ROUND WRITELN
EOLN
SIN
------
You can redefine a predeclared identifier to denote some other item.
Doing so, however, means that you can no longer use the identifier for
its usual purpose within the scope of the block in which it is
redefined.
For example, the predeclared identifier READ denotes the READ
procedure, which performs input operations. If you use the word READ
to denote something else, such as a variable, you cannot use the READ
procedure. Because you could lose access to useful language features,
you should avoid redefining predeclared identifiers.
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