Adding/ removing child nodes in existing AST

[dblommesteijn]

For a project I need some code duplication for a ruby file as described below. I'm using the parser project where it's using code from this repo. I seem to run into an issue with appending to an existing children array/ list. Perhaps someone can help me clarify how the AST::Node function updated works. Because I cannot seem to get the children list appended.

def some_function
  @a = "a"
end
def here_s_another_one
  @b = "b"
end
# result of duplicating child (what I want to achieve):
def some_function
  @a = "a"
end
def here_s_another_one
  @b = "b"
end
def some_function
  @a = "a"
end

I have found the AST::Node def updated() on line 121, but when I try to insert a copy of the example module A, It is not appended/ concatenated. Code snippet below:

file_contents = File.read(filename)
p = Parser::CurrentRuby.parse(file_contents)
src = p.children[0]
p.concat(src)
file_contents = Unparser.unparse(p)
File.write(filename, file_contents)

However, when I remove the children.to_a.freeze in the AST::Node def initialize() found here, I'm able to mutate the children list directly. Like shown below, without the modification it prints an error: can't modify frozen Array.

file_contents = File.read(filename)
p = Parser::CurrentRuby.parse(file_contents)
src = p.children[0]
p.children << src
file_contents = Unparser.unparse(p)
File.write(filename, file_contents)

PS. Without the freeze, the tests for AST seem to run perfectly (apart from should be frozen). So I wonder why you've decided to implement it like this.

[mbj]

AST::Node#updated returns an updated node. But does NOT modify in-place.

[whitequark]

Immutability of AST::Node is a deliberate design choice. Please see the documentation, as it answers the very questions you have.

I will close the issue, as freeze will stay there. If the documentation does not answer your questions, please comment further on this issue.

[dblommesteijn]

Thanks for answering my question. @whitequark is making freeze configurable an option?

[whitequark]

Out of question.

[mbj]

@whitequark full ack.

@dblommesteijn Its possible to implement all AST transforms that require mutable AST nodes with ones that work on immutable ones also. See the way the buildin AST processor works.

[dblommesteijn]

@mbj thanks, ill look into that.