[antlr-interest] Tree construction rewrite rule error

[Jim Idle]

You are just getting that because you need the rewrite at the end of an
alt or section:

| ( primary -> primary)

 ...

Jim

> -----Original Message-----
> From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-
> bounces at antlr.org] On Behalf Of Stephen Tuttlebee
> Sent: Saturday, March 05, 2011 4:08 PM
> To: antlr-interest at antlr.org
> Subject: [antlr-interest] Tree construction rewrite rule error
>
> Hi there,
>
> I'm currently using the Java.g parser grammar (by Yang Jiang) from
> http://openjdk.java.net/projects/compiler-grammar/ and am trying to add
> tree construction rewrite rules and operators to build an AST. That's
> quite a job in itself (I'm trying roughly to base it on one I found in
> the mercurial repository on that site that contained actions that
> created a javac-style AST).
>
> Anyway, my specific question relates to the LAST ALTERNATIVE in the
> following rule (from the "expression hierarchy"):
>
> unaryExpressionNotPlusMinus
>      :   TILDE unaryExpression -> ^(BITWISE_COMPLEMENT unaryExpression)
>      |   BANG unaryExpression -> ^(LOGICAL_COMPLEMENT unaryExpression)
>      |   castExpression
>      |   {inHandlerDeclaration||inRunMethodDeclaration}?=>
> inSynchronizationExpression // only allowed within handler declarations
> (which in turn are inside component definitions)
>      |   primary -> primary
>          (selector -> selector[$unaryExpressionNotPlusMinus.tree])*
>          (   PLUSPLUS -> ^(POSTINC $unaryExpressionNotPlusMinus)
>          |   SUBSUB -> ^(POSTDEC $unaryExpressionNotPlusMinus)
>          )?
>      ;
>
> The last alternative does look a little complicated but it is basically
> creating a single tree from the 'primary' in case nothing else is
> matched (due to the * and ? that mean we could not match anything),
> while the subsequent rewrite rules (->) within the alternative are
> successively building up the tree by making the tree constructed thus
> far a child of a newly created tree (this is done by referencing the
> rule itself -- referencing $unaryExpressionNotPlusMinus in the context
> of a rewrite rule means "take the current value of the tree of
> unaryExpressionNotPlusMinus").
>
> However, when I run the grammar through ANTLR (3.3) I get the following
> errors:
>
> error(100): /JavaBTranslator/src/JavaBPhase1SynSem1.g:1026:19: syntax
> error: antlr: expecting RPAREN, found '->'
>   |---> (selector -> selector[$unaryExpressionNotPlusMinus.tree])*
>
> error(100): /JavaBTranslator/src/JavaBPhase1SynSem1.g:1027:22: syntax
> error: antlr: expecting RPAREN, found '->'
>   |---> (   PLUSPLUS -> ^(POSTINC $unaryExpressionNotPlusMinus)
>
> error(10):  internal error: /JavaBTranslator/src/JavaBPhase1SynSem1.g :
> java.lang.NullPointerException
> 3 errors
>
>
> It seems that ANTLR does not like the -> that appear in the middle of
> the alternative for some reason. The problem could be a simple ANTLR
> syntax thing which I'm not clear about.
>
> Anyone have any ideas about why I'm getting these errors.
>
> Thanks,
> Stephen
>
> PS. also, it seems that coming up with the tree grammar rules to
> recognise the tree might be a little harder than constructing the tree
> in the parser, at least for the more complicated tree rewrite rules
> like this. But I'll have to cross that bridge when I come to it.
>
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