[antlr-interest] Translating expressions - advice?

Hans-Juergen Rennau hrennau at yahoo.de
Mon May 9 08:16:25 PDT 2011


Hi Bart, thank you for considering my question! Indeed, what I wrote was perhaps 
misleading. Giving the example
   (((a OR b) OR c) AND d)

I meant the result of translating the AST into text in a "canonical way", that 
is, writing this concatenation: 

formula "R"       
   : ^(operator ldefOperand rightOperand) => this string: openBracket + 
leftOperand + closeBracket + operator + openBracket + rightOperand + 
closeBracket

I suppose a deep tree created as sketched in the previous posting, that is, by 
the scheme
   : operand (operator^ operand)*

can be safely translated by applying the rule given above ("R") recursively. So 
far, so good. But the brackets are superfluous unless the current operator has a 
lower precedence than the operator in the "context", the tree level of which the 
present operand is a child. For example, this input
   A + B + C + D + E + F

generates
(((((A + B) + C) + D) + E + F)

So my question amounts to: is it a good idea to accomplish the translation in 
these steps:
a) build the AST in the standard way (meant for operation execution), creating a 
deep tree with one inner node per operator
b) serialize it using an adapted form of "R", which uses or omits the brackets 
dependent on a rule parameter providing the context operator

? Or should one build the AST differently, namely, making the top-level operands 
of an operator the children of the operator, like:
^('+' A B C D E F)

Thank you, and kind regards
-- Hans-Juergen




________________________________
Von: Bart Kiers <bkiers at gmail.com>
An: Hans-Juergen Rennau <hrennau at yahoo.de>
CC: "antlr-interest at antlr.org interest" <antlr-interest at antlr.org>
Gesendet: Montag, den 9. Mai 2011, 16:16:53 Uhr
Betreff: Re: [antlr-interest] Translating expressions - advice?

Wait I think I misunderstood. Your example `(a OR (b OR (c AND d)))` is just an 
example expression, right?
In that case, yes, these parenthesis are part of the token stream, but if you 
apply rewrite rules (or AST operators `^` and `!`) properly, these parenthesis 
are easily removed from your parse tree.

See: http://www.antlr.org/wiki/display/ANTLR3/Tree+construction
or: http://stackoverflow.com/questions/4931346/how-to-output-the-ast-built-using-antlr


Regards,

Bart.


On Mon, May 9, 2011 at 4:10 PM, Bart Kiers <bkiers at gmail.com> wrote:

I get the impression you think that when creating AST's, ANTLR inserts 
parenthesis (brackets). This is not the case: I guess what you're seeing is just 
the tree's `toStringTree()` that displays these parenthesis to make the 
hierarchy of the tree apparent.
>Or am I misinterpreting your question?
>
>
>Regards,
>
>
>Bart.
>
>


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