We can illustrate the use of the matcher to simplify the definition of a searching procedure, by redefining the procedure `findroom' introduced in chapter 1, and defined thus:
define findroom(name, list_of_lists) -> data;
;;; search list_of_lists for one starting with name
for data in list_of_lists do
if data(1) = name then
return(); ;;; i.e. stop the procedure
endif;
endfor;
;;; produce a mishap message
mishap('DATA NOT FOUND', [^name ^list_of_lists])
enddefine;
Compare this with the following:
define findroom(name, list) -> data;
vars len, breadth, height; ;;; pattern variables
if list matches [ == [^name ?len ?breadth ?height] == ]
then
[^name ^len ^breadth ^height] -> data;
else
mishap('DATA NOT FOUND', [^name ^list])
endif
enddefine;
The line
if list matches [ == [^name ?len ?breadth ?height] == ]runs the operation called `matches' with two inputs, the value of the variable `list', and the pattern on the right, which instructs the matcher what to look for in the list. It says, look for any number of elements (matched against "=="), then a list starting with the given name and containing three other things, followed by any number of elements (matched against "==" again).