Re: Question re the (non-)equivalence of Z -> z and Z -> z e (e the empty string)

Kenn Heinrich <kwheinri@bsr2.uwaterloo.ca>
13 Aug 2004 17:26:17 -0400

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From: Kenn Heinrich <kwheinri@bsr2.uwaterloo.ca>
Newsgroups: comp.compilers
Date: 13 Aug 2004 17:26:17 -0400
Organization: University of Waterloo
References: 04-08-046 04-08-058 04-08-074
Keywords: parse, theory

Larry Evans <cppljevans@cox-internet.com> writes:


> Is there something like an identity element for the alternative
> operator, |.


The alternative operator is just set union, where the sets are
languages. So the identity is the empty set, the language {}.
There is no string (including e) that matches {}.


> In addition, could this be what parsers are "in theory"
> using when the lexer's return # (as in yacc) to signal the end of input?
>


Dunno. I don't usually think of e or {} as entailing any sort of
action, but maybe you can :-)


Regards,


    - Kenn


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