Design Patterns discussion

["Robert Allan Schwartz" <notbob AT tessellation.com>]

> Parts of the grammar look like this (terminals in caps):
>
> input
>    :
>    | input COMMENT
>    | input stmt
> stmt
>    : macroassign SEMICOLON
>    | acl SEMICOLON
> macroassign
>    : MACRO EQUALS namelist <<< see below
>    | MACRO EQUALS permlist
> namelist
>    : WORD
>    | namelist COMMA WORD
>    | namelist COMMA MACRO
>
> Now, if I get a Macro node for every MACRO, how does
> a MacroCheckingVisitor distinguish between Macros on the left side of
> a macroassign, which shouldn't be checked for existence, and Macros in
> a namelist (macros need to be defined before being referenced)?

One approach is to have a boolean in the Visitor instance, which is false 
when checking is not required, and true when checking is required.
Who sets this boolean?
When the Visitor visits the macroassign node, it sets its boolean to false, 
visits the left hand side, sets its boolean to true, then visits the right 
hand side.
This boolean "plays the role of" Context in the pattern.

Hope this helps.

Robert