I have implemented a reader for a custom XML format, in Haskell. Currently, I'm using the normal 'Table' and when I convert to Latex/HTML/Docx, table cells have no borders, and no cell merging is possible. What is the simplest way to implement grid tables in my new reader?
You don't say what your custom input format is, nor how you implemented that reader.
But if you're wondering what Haskell ADT you're supposed to generate that corresponds exactly to what the markdown reader does for a specific grid table, then feed that markdown to pandoc -t native.