Backup and Retry
Multiple token-matching operators, such as $*, always try to match
the fewest number of tokens that they can. Such a
simple-minded approach could lead to problems in
matching (or not matching) classes in the LHS. For
example, consider the following five tokens in the
workspace:
A . B . C
given the following LHS rule:
R $+ . $=X $*
Because the $+
tries to match the minimum number of tokens, it
first matches only the A in the workspace. The $=X then tries to match
the B to the
class X. If this
match fails, sendmail backs up
and tries again.
The third time through, the $+ matches the A.B, and the $=X tries to match the C in the workspace. If
C is not in the
class X, the
entire LHS fails.
The ability of the sendmail program to back up and retry LHS matches eliminates much of the ambiguity from rule design. The multitoken matching operators try to match the minimum but match more if necessary for the whole LHS to match.
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