support reference to environment variables within YAML defaults files

[allefeld]

Currently I'm using

variables:
  header-includes:
    - |
      \input{\string~/.pandoc/templates/definitions.tex}

in a YAML defaults file. Note that it includes \string~, which makes LaTeX expand ~ to the home directory. I use this to access a set of (personal) mathematical macros.

It would be nicer to simply write

include-in-header: ~/.pandoc/templates/definitions.tex

but that doesn't work because Pandoc does not expand the tilde.

As @jgm noticed in #5970, having Pandoc do so would be strange since this shortcut is a quirk of Unix-like operating systems, while Pandoc is cross-platform.

Instead I propose to support reference to environment variables within YAML defaults files, for example like this:

include-in-header: ${HOME}/.pandoc/templates/definitions.tex

[jgm]

I don't think we can do this across the board, since for example one might want to include some verbatim code that uses ${HOME} in a variable. There ought to be some way to solve this problem, but it needs thought.

[allefeld]

How about enabling it only for variables that contain paths?

[jgm]

How about enabling it only for variables that contain paths?

And how would you identify these?
(Note that a variable that contains verbatim code, e.g. javascript, might contain a path with an environment variable, and you wouldn't want to mess with it in this context.)

[allefeld]

I think "variables" is not the right term. What I mean is to enable it for those entries in a defaults file (not variables: or metadata:) which take a file or directory as an argument. Meaning those that correspond to these command line options:

$ pandoc --help | egrep "FILE|DIR"
pandoc [OPTIONS] [FILES]
  -o FILE               --output=FILE
                        --data-dir=DIRECTORY
                        --metadata-file=FILE
  -d FILE               --defaults=FILE
                        --template=FILE
  -H FILE               --include-in-header=FILE
  -B FILE               --include-before-body=FILE
  -A FILE               --include-after-body=FILE
                        --highlight-style=STYLE|FILE
                        --syntax-definition=FILE
                        --reference-doc=FILE
                        --abbreviations=FILE
                        --epub-subdirectory=DIRNAME
                        --epub-cover-image=FILE
                        --epub-metadata=FILE
                        --epub-embed-font=FILE
                        --bibliography=FILE
                        --csl=FILE
                        --citation-abbreviations=FILE
                        --log=FILE
                        --print-default-data-file=FILE
                        --print-highlight-style=STYLE|FILE

That would effectively simulate what the shell does on *nix systems, but for all platforms.

[jgm]

Ah, right. This concerns certain entries in the defaults file, not variables.
Well, yes, it should be feasible to allow environment variables there.
We'd need to add a little parser for file paths that did the substitution.

[jgm]

See discussion in #5982