Add a "Free" renderer implementation of IsBlock/IsInline about commonmark-hs

[jgm]

Yes, I'm definitely open to it! I haven't really worked with free monads, so I'll have to see what it looks like in the end before deciding whether to include it here. At first glance, it seems a good idea.

[jgm]

One question is how this will handle extensibility. The way we do extensions, each extension can define additional typeclasses. Can you handle extensions that introduce new types of block or inline elements?

[lucasdicioccio]

In my intended use-case, extensibility doesn't seem to be a blocker and working on the untouched (or only supporting the basic extensions) would be enough.

I think one way to have extensibility with records that you get with typeclasses is to make a 'hole' in the structure to allow extensibility upfront (e.g., data Block ext = ThematicBreak | CodeBlock Text Text | ... | Ext ext). Then someone can implement HasThing (Block Thing) with thing = Ext (Thing = Thing1 Text | Thing2 Int Text).

A bit handwavy at this point. I'll try something for my usecase over the coming weeks and let you know here.

[jgm]

In my intended use-case, extensibility doesn't seem to be a blocker

It would be a blocker, though, for inclusion in this library.

Note: for your purposes, it might also work to just use the commonmark-pandoc classes which give you a pandoc AST. (You don't need to depend on pandoc for this, just the relatively lightweight pandoc-types.) This seems very similar to what you're looking for, though less tightly linked to commonmark's own categorizations.