Build flag to disable Lua and/or all C bits

[tarleb]

Prompted by hslua/hslua#73. Platform-independent support for hslua is sometimes tricky, and it can become an issue for porting efforts, e.g. hslua/hslua#67. Naturally, I'd rather see the underlying issues resolved, so this is only a stop-gap measure.

A flag to disable all C bits might, on the other hand, be helpful also to people trying to compile pandoc with GHCJS. That's much more involved, so disabling Lua would only be a first step.

[jgm]

See #4535 - there's a list there of all the cbits in our transitive dependencies. It's hopeless to disable them all.

[jgm]

Perhaps it's worth considering a flag to disable lua support. However, I believe it's considered bad practice to have your API change depending on the settings of cabal flags. Maybe it could be possible to keep the same API but raise an exception if lua support is not compiled in?

[tarleb]

I've played with this for a bit. The only way to make this work is to move all occurrences of the Lua type inside Text.Pandoc.Lua, to never expose a function with type Lua in its signature, and to stop exposing module Text.Pandoc.Lua. This is simple enough, but it seems that this would deprive library users of useful functionality. On the other hand, library users write Haskell instead of Lua to extend pandoc.

If the goal is to keep exposing "raw" pandoc Lua functionality, then one solution could be to leverage finer-grained library packages. E.g., having a pandoc-core package would make it possible to write a pandoc-core-lua and pandoc-core-nolua packages, which would then be used by pandoc proper. Cabal 2 supports having multiple libraries in the same cabal file, and finer grained modularity could solve a lot of portability issues. But there are obvious and undeniable costs in complexity and maintenance to this approach.

The issue seems to be of low priority now that the hslua cross-compilation issue has been resolved. Should we close this for now in favor of #4535?

[jgm]

I'm in favor of closing this if there isn't an urgent need for it.