We do not always want to know what the matching item was. Sometimes we will know in advance that an item matching the pattern is present in the DATABASE and want only the value of some fragment of the item.
For this purpose the procedure LOOKUP is provided. It does not return <TRUE> or <FALSE> but merely sets the value of queried pattern variables when a match is found, and causes an error if no match is found.
alladd([[a b c d] [d c b a] [a b d c]]);
lookup([= ?x b ==]);
x =>
** c
In the event of no match being found an error will result
lookup([?x == e]);
;;; MISHAP - LOOKUP FAILURE
;;; INVOLVING: [? x == e]
;;; DOING : sysprmishap mishap lookup ......
Note that PRESENT is to MATCHES as LOOKUP is to -->. PRESENT and MATCHES
check a condition and return a true/false result, and may simultaneously
bind pattern variables. But only PRESENT searches over a list. LOOKUP
and --> do not return any result, and only LOOKUP searches.
procedure returns iterates over mishap on
a boolean the database failure
---------------------------------------------------------
matches yes no no
--> no no yes
present yes yes no
lookup no yes yes