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New Features of Eli Version 4.2

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New token processor lexerr

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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