[antlr-interest] tree transformation question (if-elif-else -> if-else)

[Bryan Ewbank]

I solve this by building the tree differently.  I think you are using
a flat grammar to consume these - perhaps something like this:

  if_s   : "if"^ expr statements ( elif_s )* ( else_s )? ;
  elif_s : "elif"^ expr statements ;
  else_s : "else"! statements ;

I use a recursive grammar instead, drilling into the elif_s production
to build the tree.  It produces the tree you want, and recognizes the
same language (if I wrote it correctly):

  if_s   : "if"^ expr statements ( elif_s )? ;
  elif_s : "elif"^ expr statements ( elif_s | else_s )? ;
  else_s : "else"^ statement ;

The resulting tree looks like this (again, if I wrote the above correctly):

   #( "if"
        expr
        statements
        #( "elif"
             expr
             statements
             #( "else"
                  statements
             )
         )
     ) 

On 7/1/05, Klaas Hofstra <antlr at klaashofstra.com> wrote:
> Hi,
> 
> I have a problem with tree transformations that must have been solved before
> because it seems to me that it occurs frequently when converting one language
> to another.
> 
> I need to convert the following pseudo-code:
> 
> if expression
>   statements
> elif expression
>   statements
> elif expression
>   statements
> else
>   statements
> 
> into:
> 
> if expression
>   statements
> else
>   if expression
>     statements
>   else
>     if expression
>       statements
>     else
>       statements
> 
> Translated into trees, I need to transform the tree :
> 
> tree1 =
> (IFBLOCK  (IF  E1  S1) (ELIF  E2  S2) (ELIF  E3  S3) (ELSE  S4))
> 
> into:
> 
> tree2 =
> (IFBLOCK  (IF  E1  S1)  (ELSE  (IFBLOCK  (IF E2 S2)  (ELSE
> (IFBLOCK  (IF E3  S3) (ELSE  S4))))))
> 
> , where E=EXPRESSION, S=STATEMENTS
> 
> My problem is that the rewrite rule for ELIF  is something like:
> 
> (ELIF  E  S) -> (ELSE  (IFBLOCK  (IF E S)  subtree_of_next_elif_or_else)
> 
> When traversing the tree from left to right with a treeparser,
> 'subtree_of_next_elif_or_else' is not available when it is needed.
> 
> How can this problem be solved?
> 
> Thanks in advance,
> 
> Klaas
> 
>