Hello People,
being an ANTLR beginner, I would very much appreciate advice concerning good
practise for a rather simple task. The task is the translation of a JPQL's
(Java
Persistence Query Language) "where clause" into a proprietary query language.
The clause has the well-known expression structure: operands conncected by
three
operators: OR, AND and NOT, where precedence increases in that order. Example:
a.x='1' AND (a.y='2' OR b.z='3') AND a.v like 'abc%'
An important point is that the translation result will have a similar
structure,
that is, it will also be operands connected by those operators. Example:
x='1' AND (y='2' OR z='3') AND v='123*'
For this reason I am not sure if the "classical" approach for dealing with
left-associative operators, as shown in the "Definitve ANTLR Reference" (3. A
quick tour...) is the most appropriate one in this case. I mean rules like:
conditional_term ('OR'^ conditional_term)*
conditional_factor ('AND'^ conditional_factor)*
This creates deep trees, where each operator creates a new level. That is fine
for processing the operations. But a straightforward translation of the tree
into a similar sequence of operands and operators yields a result which is
correct but can be ugly, due to superflous brackets, example:
(a OR (b OR (c AND d)))
One possibility is to process the tree, removing superfluous brackets - perhaps
by passing the "context operator" into the rule as a parameter, so that the
rule
can decide if to create brackets or not. This should not be too difficult, but
my question is: is there a good practise for accomplishing the task? Would you
recommend the approach just sketched, or a different tree representation to
start with?
(A tree I do want because there are other parts to be translated, not only the
where clause, and a tree seems to me the way to deal with (possibly yet
growing)
complexity.
Thank you very much for any suggestions.
-- Hans-Juergen
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