HELP PERCENT Steven Hardy Mar 1982
The percent symbol, %, has a number of different uses in POP-11. (For a
detailed tutorial introduction, see TEACH *PERCENT.)
CONTENTS - (Use <ENTER> g to access required sections)
-- Use 1: Evaluating structure constants
-- Use 2: Partial application
-- Use 3: Dynamic local expressions-- Use 1: Evaluating structure constants ------------------------------
The first use is in structure constants. Normally, structure constants
are unevaluated, so, for example:
[the sum of 3 and 4 is 3 + 4] =>
** [the sum of 3 and 4 is 3 + 4]
Surrounding part of a structure constant with percents causes it to be
evaluated, for example:
[the sum of 3 and 4 is %3 + 4%] =>
** [the sum of 3 and 4 is 7]
Any number of expressions can be surrounded by percents, but they must
then be separated with commas. So, for example, the following two
expressions are equivalent:
[%x% %y%]
[%x, y%]
Whereas
[%x y%]
will produce the same syntactic error message as
x y
If an expression surrounded by percents produces more than one result
(i.e. it leaves more than one thing on the stack), then all the results
go into the structure, for example:
[%10 // 3%] =>
** [1 3]
Percents may be used in vector expressions as well as list expressions:
{a vector with % 3 + 2 % items } =>
** {a vector with 5 items}
And with -cons_with-, as in the following expression to construct a
string of characters:
vars char;
cons_with consstring {%for char from 65 to 75 do char endfor %} =>
** ABCDEFGHIJK
(See HELP * ASCII for the representation of character codes).
For more examples of the use of % see TEACH * PERCENT.
If the percent symbol is used in a string expression it is taken as
part of the string:
'a b c % d e f % g h' =>
** a b c % d e f % g h
See also
HELP *LISTREAD - reads in lists without evaluating constants
-- Use 2: Partial application -----------------------------------------
The second use of percents is to provide partial application. The
expression:
foo(%a, b%)
creates a 'closure' of -foo-, that is a new procedure in which the last
two arguments of -foo- have -a- and -b- as 'frozen' values. The same
thing is done by the following:
partapply(foo, [%a, b%])
or
consclosure(foo, a, b, 2)
See also
HELP *CLOSURES --- A summary of closures in POP-11
HELP *PARTAPPLY --- A summary of partial application in POP-11
REF *PROCEDURE --- Details on procedures in POP-11
-- Use 3: Dynamic local expressions -----------------------------------
Percent symbols can be used with -dlocal- to indicate actions to be
taken on procedure entry and exit, including abnormal entry or exit such
as use of -chain-, or running or resuming processes. For further details
see HELP * DLOCAL
--- C.all/help/percent -------------------------------------------------
--- Copyright University of Sussex 1988. All rights reserved. ----------