Local data dir(s) at a higher precedence than --data-dir

[bpj]

The problem

The --data-dir option is useful, but its overriding the default user data directory is often a problem, since you may want to use files both from a custom data directory and the default directory, particularly filters and defaults files.

Proposed solution

I think the best solution is a command line/defaults file option --local-data-dir used to specify one or more directories (with appropriate subdirectories) where Pandoc will look for data files after looking in the current directory but before looking in the --data-dir. (preferably repeatable)

Alternative solutions

• Keep everything in the current directory. :-( This is not only messy but is suboptimal for example if you have a filters etc. which you want to be able to use from different places in a directory tree, but still cannot or don’t want to keep in the default data directory, e.g. if they have standardized (short) names.
• Use long paths repeatedly on the command line. :-(
• Use custom prefix environment variables (which need to be defined per session and/or on several accounts/machines and cannot be used in defaults files.
• Look for directories with a certain name, say pandoc-data in the current directory and its ancestors. This approach has its own well-known problems!

Related

• Look for data files of all kinds in data dir subdirectories #10251
• Expand ${HOME}, ${USERDATA} and ${.} In all strings in defaults files #10252

[jgm]

The user data directory is already "local," in the sense that it provides the opportunity to override system defaults. So, what you're proposing is having three layers -- the system defaults, the user data directory, and a super-local data directory.

This would add a lot of complexity.

One solution you didn't mention:

• use symlinks in the custom data directory to link back to the regular data directory for things that are needed there.