New Features of Eli Version 4.2
 
The token processor lexerr reports that the character sequence is
not a token. It does not alter the initial classification, and does not
compute a value.
Normally, a lexical analyzer generated by Eli attaches an error report
to each character that it does not recognize. While this behavior is
adequate in most cases, it is sometimes necessary for the designer to
specify a particular sequence of characters to be erroneous. One
typical example is disallowing tab characters:
TAB [lexerr]
The canned description TAB handles all of the coordinate updating
(see Maintaining the source text coordinates of Lexical Analysis). Since there is no label on this line, the tab
character is classified as a comment. That classification is not
changed by lexerr, which simply reports a token error at the
coordinates of the tab
(see Making White Space Illegal of Lexical Analysis).
There is no source file for lexerr; it is a component of the
scanner itself, but its interface is exported so that it can be used by
other modules.
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