Stropping (syntax)

When applied to computer languages, stropping refers to the method used to mark letter sequences as having a special property such as being a keyword or certain type of variable or storage location.

The use of stropping allows the same letter sequence to be used both as a keyword and as an identifier, and more efficient lexical pre-pass before parsing, virtually eliminating the need for a complex lexer such as Lex.

Syntaxes

A range of different syntaxes for stropping have been used:

• Algol 60 commonly used only the convention of quotes around the word (e.g. 'BEGIN' or ‘BEGIN’).
• Algol 68^[1][2] in some implementations treat letter sequences prefixed by a single quote, ', as being keywords (e.g., 'BEGIN)^[3]

In fact it was often the case that several stropping conventions might be in use within the one language:

• Atlas Autocode had the choice of three: keywords could be underlined using backspace and overstrike on a Flexowriter keyboard, they could be introduced by a %percent %symbol, or they could be typed in UPPER CASE with no delimiting character (in which case all variables had to be in lower case).
• ALGOL 68RS allowed the use of several even within the one language processor.

Examples of different ALGOL 68 styles

¢ bold or underline typeface ¢
mode xint = int;
xint sum sq:=0;
for i while
  sum sq≠70×70
do
  sum sq+:=i↑2
od 

'pr' quote 'pr' 
'mode' 'xint' = 'int';
'xint' sum sq:=0;
'for' i 'while'
  sum sq≠70×70
'do'
  sum sq+:=i↑2
'od'

.PR UPPER .PR
MODE XINT = INT;
XINT sum sq:=0;
FOR i WHILE
  sum sq/=70*70
DO
  sum sq+:=i**2
OD 

.PR POINT .PR
.MODE .XINT = .INT;
.XINT SUM SQ:=0;
.FOR I .WHILE
  SUM SQ .NE 70*70
.DO
  SUM SQ .PLUSAB I .UP 2
.OD

.PR RES .PR
mode .xint = int;
.xint sum sq:=0;
for i while
  sum sq≠70×70
do
  sum sq+:=i↑2
od 

Modern languages

Most modern programming languages do not use stropping - although Ruby and Perl both use sigils to identify characteristics of variables/constants: Perl to designate the type of variable, Ruby to distinguish variables from constants and to indicate scope.

See also

• Escape character
• Sigils

References

• A. van Wijngaarden; et al. (1976). Revised Report on the Algorithmic Language ALGOL 68. Springer-Verlag. ISBN 0-387-07592-5. OCLC 1991170. 
• C. H. Lindsey; S. G. van der Meulen (1977). Informal Introduction to ALGOL 68. North-Holland. ISBN 0720407265. OCLC 230034877. 
• W. J. Hansen; H. J. Boom (1978). "Report on the Standard Hardware Representation for Revised ALGOL 68". Acta Informatica 9: 105–119. doi:10.1007/BF00289072.