Using myst_parser's new "substitutions" feature

Describe the bug

I’m trying to get the new “substitution” directive from myst_parser v0.13 and running into some trouble translating the “conf.py” file to jupyterbook’s “_config.yml”.

Problem 1: dependency issues

Trying to upgrade myst_parser to v0.13 caused some issues with dependencies for myst-nb

ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
myst-nb 0.10.1 requires myst-parser~=0.12.9, but you have myst-parser 0.13.0 which is incompatible.
jupytext 1.6.0 requires markdown-it-py~=0.5.2; python_version >= "3.6", but you have markdown-it-py 0.6.0 which is incompatible.

I think this was just a warning, and I ended up upgrading myst_parser anyway.

Problem 2: myst_enable_extensions

The Myst docs say:

To enable all the syntaxes explained below:

myst_enable_extensions = [
    "amsmath",
    "colon_fence",
    "deflist",
    "dollarmath",
    "html_image",
    "linkify",
    "replacements",
    "smartquotes",
    "substitution"
]

What is the way to enable these in jupyterbook? Here is my attempt in _config.yml:

# bunch of stuff that's not relevant here

sphinx:
  config:
    html_show_copyright: false
    html_extra_path: ['files']

parse:
  myst_extended_syntax: true

myst_enable_extensions: 
  - amsmath
  - colon_fence
  - deflist
  - dollarmath
  - html_image
  - linkify
  - replacements
  - smartquotes
  - substitution

myst_substitutions:
  course: 'THISISAMAZING1010'

Problem 3: Using substitutions

I imagine that with the _config.yml file above, this should work in any md file:

This is the official syllabus and course outline for COSC 123 {{ course }}.

… but {{ course }} doesn’t replace the text as I expected.

I also tried to add the substitution definition at the top of the file as “front-matter” using three dashes, but that doesn’t work either (no error, just leaves it as {{ varname }}.

E.g.,

---
substitutions:
  course2: "I'm a substitution"
---

To Reproduce

See above.

Expected behavior

{{ course }} should be replaced with “THISISAMAZING1010”

Environment

• Python Version [e.g. 3.7.1]: 3.8.3
• Package versions or output of jupyter-book --version:

Jupyter Book: 0.9.1
MyST-NB: 0.10.1
Sphinx Book Theme: 0.0.39
MyST-Parser: 0.13.0
Jupyter-Cache: 0.4.1
NbClient: 0.4.1

• Operating System: macOS 11.1

Additional context

RE: Problem 2, I see the same warnings as seen in issue #1152…

[chrisjsewell]

You can already pass in directives, the text is parsed before it is substituted in

[chrisjsewell]

you should actually just be able to replace {{FORUM_LINK | replace('JOIN_LINK', ED_ID)}} with {{FORUM_LINK.replace('JOIN_LINK', ED_ID)}}