Global Katex \newcommand

[fdegrave]

First of all, thank you for making my teacher's life easier! I use Marp a lot for math notations, and I often use things like:

$$\newcommand{\cons}[2]{\mathtt{cons}\ #1\  #2}
...
$$

It works perfectly, but I need to redefine all my commands in each KaTeX environment, they are not "kept" for the rest of the document. Is there a way to define my \newcommand only once in the .md document, and use them anywhere after?

[yhatt]

Currently each KaTeX renderings in Marp Markdown have individual contexts so \newcommand is working only in a defined environment. Fortunately there is a way to persist defined commands in KaTeX API, so we might move for that if received many requests.

Workaround for Marp CLI

Macros can also be defined in the KaTeX rendering options.

-- https://katex.org/docs/supported.html#macros

Our core engine can customize KaTeX rendering option, and Marp CLI can modify it by using the configuration file. Try using this configuration if you are using Marp CLI:

.marprc.json

{
  "options": {
    "math": {
      "katexOption": {
        "macros": {
          "\\cons": "\\mathtt{cons}\\ #1\\  #2"
        }
      }
    }
  }
}

[fdegrave]

Thank you! However I use Marp in VSCode, is there a way to provide the same option through the settings.json?

[yhatt]

No. Marp for VS Code has no way to set advanced option because allowing the advanced configuration like that means to open a security hole. We are responsible for saving users from any malicious. Please use Marp CLI.

[yhatt]

[UPDATED]

Hey @fdegrave, I've updated Marp's math handling in marp-team/marp-core#212, to persist KaTeX macros, that have defined as global, between math typesetting in a Markdown. It has shipped as Marp Core v1.4.1, and that has bundled into the latest update of Marp tools, CLI v0.23.1 and VS Code extension v0.17.1!

KaTeX has supported following commands to define new command as globally:

\gdef, \xdef, \global\def, \global\edef, \global\let, and \global\futurelet will persist between math expressions.

-- https://katex.org/docs/supported.html#macros

As you can see from this quote, please notice that \newcommand you're using cannot persist the command.

[fdegrave]

Great! Thank you so much! Well noted for the unsupported \newcommand, I'll manage ;-)