Pandoc patch for obsidiansystems/obelisk

This is a patch to Pandoc for use as Haskell library with Obsidiansystems Obelisk.

This isn't tested by any means, but seems to be working fine.

What this patch does

The skylighting library is fixed to the very old version 0.9. This is necessary because new versions introduce a C-dependency via xml-conduit.

aeson is downgraded to version 1.5.6.0 with hopefully no big consequences.

The writer for ConTeXt is completely removed. It depends on features of the new skylighting versions.

How to use

See learn-palantype for a project that uses Pandoc with this patch, especially check the default.nix. In default.nix all versions that needs pinning are pinned.

Why is a patch necessary

The obelisk framework allows to build web applications (and smartphone apps) using Haskell only, implementing reflex-frp for interactive UIs.

Obelisk projects are split into three cabal packages: frontend, common, and backend. The frontend and common packages are compiled by both, GHC and GHCJS. The latter one is necessary to have Haskell programs running inside browsers.

Obelisk is also stuck with GHC 8. The latest release uses GHC 8.6.4, the development branch uses GHC 8.10.7.

Using Pandoc with Obelisk

The use of any library with an obelisk project needs to adapt to these compilers. backend dependencies need to work with the older GHC version, which usually isn't a big deal. Dependencies to frontend and common additionally can't depend on C compilers. And those dependencies on C sometimes sneak in and ruin your day.

For reflex-frp, there is reflex-dom-pandoc, which allows you to use Pandoc's document representation Pandoc as input to a reflex component elPandoc, effectively rendering Pandoc to HTML.

reflex-dom-pandoc has a dependency to pandoc-types (not Pandoc), which is great: Having frontend or common depend on the whole Pandoc package would cause all kinds of problems. Instead, I opt to leave the Pandoc-dependency with backend (and thus any document conversions happend on the server), while the frontend only renders the Pandoc type.

The technical problems

There is a choice to be made between Pandoc versions. The newest version not only has the latest features, but also, all dependencies to Lua have been separated and are not part of Pandoc the haskell library.

An older Pandoc version would potentially work out of the box with one of the GHC versions that are demanded by obelisk (8.6.4 or 8.10.7), but I opted to make Pandoc 3.1 work with GHC 8.10.7 as it turned out not to be a big deal.

For pandoc-types, there is a C-dependency, unfortunately, that we need to get rid of.