[antlr-interest] Translating expressions - advice?

[Loring Craymer]

Optimal placement of parentheses is tricky; as a first step, you want to use the 
form
^('+' A B C D E F )
as long as the operator is associative;
then you want to parenthesize only when required to by changes in operator 
precedence; the easiest way is actually to have two versions of each expression 
rule, one for top level invocation and one for nested invocation (the nested 
versions parenthesize).

--Loring



----- Original Message ----
> From: Hans-Juergen Rennau <hrennau at yahoo.de>
> To: Bart Kiers <bkiers at gmail.com>
> Cc: "antlr-interest at antlr.orginterest" <antlr-interest at antlr.org>
> Sent: Mon, May 9, 2011 8:16:25 AM
> Subject: Re: [antlr-interest] Translating expressions - advice?
> 
> 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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