How can I check if a file exists in the render directory?

[coatless]

Description

I need to check whether a specific file exists in the Quarto render directory before proceeding with further actions. While I can override the file on each render, I would like to try to preserve it as users may modify it or substitute in their values.

Given this conundrum, I'm wondering if it is okay to list the resource under contributes instead of trying to register it with quarto.doc.add_supporting()/quarto.doc.add_format_resource(path)/quarto.doc.add_html_dependency(), e.g.

contributes:
  filters:
    - filter.lua
  resources:
    - config.js

https://quarto.org/docs/extensions/filters.html#filter-extensions

I only saw one example of using a resource-like key inside of the dev article on Custom formats.

[cscheid]

I need to check whether a specific file exists in the Quarto render directory before proceeding with further actions.

Where is this check happening, and what are the further actions?

[coatless]

Side question:

For a single Quarto render session, is there a way to "flag" or store between applications of the Pandoc Lua filter that this process has already been completed?

[coatless]

@cscheid Awesome! I think this was just added in #8525

I think from Lua, I can access this information with:

os.getenv("QUARTO_PROJECT_ROOT")

Any thoughts on the side question of being able to pass a state between applications of a filter?

[cscheid]

For a single Quarto render session, is there a way to "flag" or store between applications of the Pandoc Lua filter that this process has already been completed?

I'm pretty sure the answer is no, because we have to run the filters every time for every document (they walk the AST and change the document, we can't assume "it's already been completed": it needs to happen every time.)

But I don't know what's "this" in "this process" in your original sentence, so I might be missing something.

[coatless]

@cscheid I'm trying to reduce needing to re-check if a configuration is present and potentially re-use a loaded config instead of re-reading from disk.

The goal would be to only execute the following process once per quarto render on a project, not per-Qmd document.

1. Check for existence of config.js/config.toml/config.json
2. If found then read in portions of the config:
   • check if config matches latest version
     • otherwise pull from template and move existing config to backup.
3. If not found then:
   • copy from template into working directory via either quarto.doc.add_supporting()/quarto.doc.add_format_resource(path)
4. Retrieve values from config

[cscheid]

Ok, so Lua filters are only ever called on a per-document basis. So if your document has filters: ... in it, that filter is necessarily (and always) called at every document render, because the goal of filters is to change documents.

If you have pre-render or post-render scripts that are implemented in Lua, then those are not filters, and those get executed once at the beginning or end of a project render.

However, your comment is talking about quarto.doc.add_supporting(), and that call doesn't make sense from the perspective of a project, because (for example) those directories can change depending on the format being rendered, and the format is a document option, not a project option.

So I think that what you have to do is a combination of things:

• in the project pre-render script, you check for the presence of the configuration and ensure that it exists in a path that's accessible by every document render. Call that path your "cache"
• in a document filter, you always use quarto.doc.add_supporting() getting the values from your cache, and so on.

Does that make sense?