[Pyparsing] Get Better Error Messages - Prevent Backtracking

[Eike W. <eik...@gm...>]

I want to propose an other way, how to get better error messages: 
Prevent backtracking at certain points in the grammar. I have 
attached a simple example implementation that works with Pyparsing 
1.4.5.

Often it is clear that, when a certain parser fails, there is surely 
an error in the input. Backtracking in this situation is bad, because 
information about the location and the cause of the error might be 
lost. A common situation is after a keyword.

Imagine a programming language, where you define variables like this:
    data a, b, c: Real;

After the 'data' keyword there must be a list of variable names, a ':' 
character, the name of a type and a ';' character. If this pattern 
does not appear after the 'data' keyword there is an error in the 
program.  

(See lines 46, 47 in the example program.)
A usual parser for this statement would look like this:
    dataDef1 = Group(Keyword('data') 
                     + attrNameList + ':' + identifier + ';')

I propose the 'ErrStop' class, an additional 'ParserElement', that 
stops parsing when a parser given to it fails. It is used like this:
    dataDef2 = Group(Keyword('data') 
                     + ErrStop(attrNameList + ':' + identifier + ';'))                       

In the example program the 'data' statement is combined with two 
additional statements, 'foo1;' and 'foo2;' to form a programming 
language. The program output shows that using the 'ErrStop' class 
really preserves more information about the error (missing comma 
after position 15). Additionally the parse result of a successful run 
is not altered.


The program output:
Test regular parser:
[['foo1', ';'], ['data', 
['a', ',', 'a1', ',', 'b'], ':', 'Real', ';'], ['foo1', ';']]
Expected end of text (at char 6), (line:1, col:7)

Test parser with backtracking stop:
[['foo1', ';'], ['data', 
['a', ',', 'a1', ',', 'b'], ':', 'Real', ';'], ['foo1', ';']]
Expected ":" (at char 17), (line:1, col:18)