Conceal Support for 2-Command Arguments

[EmperorEntropy]

Is your feature request related to a problem? Please describe it.
My feature request is related to an interesting article I read on Unicode Plain Tex Encoding for math. One of the unicode "simplifications" intrigued me, and it was how \binom{n}{k} was simplified into (n¦k). I would like to have a similar functionality with Vimtex's conceal system.

Describe the solution you'd like
To be precise, I would like conceal support for 2-command arguments, which are arguments that look like \cmd{}{}. Perhaps a command like g:vimtex_syntax_custom_cmds_with_two_args would work? For the example I want, it could look like the following

let g:vimtex_syntax_custom_cmds_with_two_args = [
          \ {'name': 'binom',
          \  'mathmode': 1,
          \  'cchar_open': '(',
          \ 'cchar_mid': '¦',
          \  'cchar_close': ')'},
          \]

where cchar_open is inserted before the first argument, cchar_mid is the char inserted between the two arguments, and cchar_close is inserted after the second argument. So, something like \binom{4}{2} would appear as (4¦2).

Ideally, cchar_open and cchar_close would be optional, allowing for cases where we don't need them as part of the concealment. A basic example would be fractions, and possible customization for it might look like:

let g:vimtex_syntax_custom_cmds_with_two_args = [
          \ {'name': 'frac',
          \  'mathmode': 1,
          \ 'cchar_mid': '/'},
          \]

In this case, \frac{2}{3} could become 2/3 under concealment.

This seems rather complex to implement, but hopefully something like this would be possible to have. It would make concealment be a lot more useful. Thank you for considering this suggestion!

[lervag]

Sorry for the slow response here. First, let's consider how we could write such a rule. This fits very nicely into the "very hacky" type solutions. :)

I came up with this (still in draft mode). It seems to work:

syntax match texMathCmdCBinom "\v\\binom>"
      \ skipwhite nextgroup=tex1
      \ conceal cchar=(
syntax cluster texClusterMath add=texMathCmdCBinom
syntax region tex1
      \ matchgroup=tex1CD
      \ concealends cchar=|
      \ start="{" skip="\\\\\|\\}" end="}"
      \ skipwhite nextgroup=tex2
      \ contained contains=@texClusterMath
syntax region tex2
      \ matchgroup=tex1CD
      \ concealends cchar=)
      \ start="{" skip="\\\\\|\\}" end="}"
      \ contained contains=@texClusterMath

I'm thinking though that perhaps it is better not to make an option for this. Instead, perhaps we can add a VimTeX api function that anyone could use in their after/syntax/tex.vim with a proper specification? That might be a better solution for the current g:vimtex_syntax_custom_cmds_with_concealed_delims as well. Not sure, though...

[lervag]

I'm pushing a function that should work, please test it. You 1) update vimtex, then 2) add the following to your after/syntax/tex.vim (or similar):

  call vimtex#syntax#core#new_cmd_with_concealed_delims_2args({
        \ 'name': 'binom',
        \ 'mathmode': 1,
        \ 'cchar_open': ')',
        \ 'cchar_mid': '|',
        \ 'cchar_close': '(',
        \})

[lervag]

Scratch that. I updated the existing function instead. See :help vimtex_syntax_custom_cmds_with_concealed_delims and let me know if it does not work as expected.