Grammar for optional elements

Mohitz <coolmohitz@gmail.com>
Tue, 12 Jun 2007 02:21:31 -0700

          From comp.compilers

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From: Mohitz <coolmohitz@gmail.com>
Newsgroups: comp.compilers
Date: Tue, 12 Jun 2007 02:21:31 -0700
Organization: Compilers Central
Keywords: parse, question, comment

Hi,


I need to create a parser for a language something like this.


attribute1: value;
attribute2: value;
attribute3: value;


All the attributes are optional but can occur only once...


One way is to specify the grammar as follows: This doesn't do the
'SINGLE occurence check'
Can a grammar be designed which ensures that each attribute can be
entered only once.


attribute_list : attribute_list attribute | attribute
attribute : ATTRIBUTE1 ':' IDENTIFIER | ATTRIBUTE2 ':' IDENTIFIER |
ATTRIBUTE3 ':' IDENTIFIER


[You can do it but you'll have a huge ugly grammar. You're much
better off with a grammar that accepts any list of attributes, and
reject duplicates in your semantic code. -John]


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