cross-posting 
of https://stackoverflow.com/questions/67109880/is-it-posible-to-enable-extensions-on-pandoc-filters

Why do you prefer to set this in the filter instead of on the writer? I 
don't think there's a way to do that... because the filter just outputs an 
AST again, and what you're trying to change is happening in the writer...

On Thursday, April 15, 2021 at 4:20:32 PM UTC+2 Angel Joaniquet wrote:

> The lua split function is this
>
>
> ```lua
> function split(str,pat)
>    local tbl = {}
>    str:gsub(pat, function(x) tbl[#tbl+1]=x end)
>    return tbl
> end
> ```
>
> El jueves, 15 de abril de 2021 a las 16:08:23 UTC+2, Angel Joaniquet 
> escribió:
>
>>
>> I'm trying to make a filter to transform some features of org-mode to 
>> GitLab-markdown (not supported by Pandoc out of the box), in particular, 
>> the math blocks.
>>
>> The filter should work when transforming to `markdown`, but instead of 
>> giving the markdown format for the math blocks (enclosed by `$$...$$`), it 
>> should write the blocks as 
>>
>>     ```  math
>>     a + b = c
>>     ``` 
>>
>> The preces I have now is 
>>
>> In org-mode, the math blocks are simply the latex code:
>>
>> ```
>> \begin{equation}
>> a + b = c
>> \end{equation}
>> ```
>>
>> this is parsed as a pandoc AST `RawBlock` with format `latex`. I then 
>> remove the first (`\begin{equation}`) an last line (`\end{equation}`), and 
>> construct a pandoc `CodeBlock` with atrributes `{"math"}`, so the 
>> `CodeBlock` object displays in AST as
>>
>> ```
>> CodeBlock ("",["math"],[]) "a + b = c\n"
>> ```
>>
>> and then I let Pandoc create the markdown document, and the written 
>> result is 
>>
>>     ``` {.math}
>>     a + b = c
>>     ``` 
>>
>>
>>
>> **The question:**  
>> I want the bare `math`, not `{.math}` written, without the use of CLI 
>> options. 
>>
>>
>> I am aware that this can be done setting the Writer extension 
>> `fenced_code_attributes` to false (eg. `$pandoc -w 
>> markdown-fenced_code_attributes ...`), but I would much prefer this done 
>> inside the filter. 
>>
>> Or is it possible to set the extensions inside the filter?
>>
>>
>> Here is my atempted lua-filter:
>>
>> ``` lua
>> function RawBlock(rb)
>>    if rb.format == "latex" then
>>       local text = rb.text
>>       split_text =  split(text, "[^\n]*")
>>       if split_text[1] == '\\begin{equation}'  and 
>> split_text[#split_text-1] == '\\end{equation}' then
>>          table.remove(split_text, #split_text-1)
>>          table.remove(split_text, 1)
>>          text = table.concat(split_text, "\n")
>>          local cb = pandoc.CodeBlock()
>>          cb.attr = {"",{"math"}}
>>          cb.text = text
>>          return cb
>>       end
>>    end
>> end
>> ```
>>
>>
>> Kind regards, 
>>
>> Angel
>>
>>
>>
>>

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