Stropping

When applied to computer languages, stropping refers to the method used to mark letter sequences as having a special property (most often being a keyword or certain type of variable/storage location). For instance, some implementations of Algol 68^[1][2] treat letter sequences prefixed by a single quote, ', as being keywords (e.g., 'BEGIN)^[3] whereas Algol 60 commonly used only the convention of quotes around the word (e.g. 'BEGIN' or ‘BEGIN’). In fact it was often the case that several stropping conventions might be in use within the one language, and sometimes even within the one language processor, such as ALGOL 68RS. The Atlas Autocode compiler 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).

The use of stropping allows the same letter sequence to be used both as a keyword and as an identifier. Most modern programming languages do not use stropping. Exceptions include Ruby and Perl, both of which use sigils to identify characteristics of variables/constants (Perl uses sigils to designate the type of variable, Ruby uses it to distinguish variables from constants and scope). As well as increasing the set of identifiers available, stropping allowed for a much more efficient lexical pre-pass (commonly called 'line reconstruction') before parsing, virtually eliminating the need for a complex lexer such as Lex.

Example of different ALGOL 68 program representations

¢ bold/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 

See also

• Escape character

References

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.