Circular Dependency Loading

[ET]

I am using treetop to construct a grammar but am having problems with
what seems to be a circular dependency.

Consider the following structure:

test.rb
lib/A.treetop
lib/B.treetop
lib/AB.treetop
lib/all.rb

And the sources for the files

# test.rb
require File.dirname(__FILE__) + "/lib/all"

# lib/A.treetop
grammar A
include AB
end

# lib/B.treetop
grammar B
include AB
end

# lib/AB.treetop
grammar AB
include A
include B
end

# lib/all.rb
require 'treetop'

require File.dirname(__FILE__) + '/A'
require File.dirname(__FILE__) + '/B'
require File.dirname(__FILE__) + '/AB'

It's very obvious that I've introduced a circular dependency.

Now when I run test.rb

ruby test.rb

I receive the following error:

/home/eric/.rvm/gems/ruby-1.9.2-rc2/gems/treetop-1.4.8/lib/treetop/
compiler/grammar_compiler.rb:42:in `class_eval': uninitialized
constant A::AB (NameError)
....

meaning that it could not include the 'AB' grammar in the 'A' grammar
since it's after it in the 'all.rb' file.

However, this circular dependency is necessary.

Consider this pseudo-grammar where it would be applicable:

# Statement file
include ForStatement
include SwitchStatement
statement -> for_statement / switch_statement / other_statement

# ForStatement file
include Statement
for_statement -> 'for (' expr ')' statement

As you can see, the for loop can contain other statements inside it.
But I cannot accomplish this unless all the rules are in the same
file.

Can someone please point me in the right direction on how to
accomplish this if it's even possible?

[Clifford Heath]

On 15/10/2010, at 2:11 PM, ET wrote:
> However, this circular dependency is necessary.

I don't believe that's true. You need to define 'statement' in one
grammar, and that grammar needs to include all the modules
that provide statements.

A grammar may call rules which it doesn't either include or define,
so this is perfectly acceptable. Your "for" statement grammar can
call "statement" without defining it or including a module that defines
it.

However, another feature of which you might not be aware is that
an included grammar may override a rule in the grammar into which
it's included, and can call "super" to invoke the overridden rule.

So if I have a grammar which defines 'statement' with five alternatives,
and I want to include a grammar which defines a sixth alternative (say
an "until" statement), I can say:

rule statement
until_statement / super
end

in my included grammar. This first tries for an until_statement, then
falls back to the base grammar's "statement" rule.

Clifford Heath.

[ET]

Ahh... this makes good sense. Thanks Clifford.