It would be useful to have a function that invokes a callback function for each maximal clique found.
There is already such a function, igraph_i_maximal_cliques, but it is not publicly exposed. I don't know if it would be okay to expose this and how pull request #824 would affect it.
Pull request #888 adds such a function for all (not just maximal cliques). This feature request is specifically for maximal cliques. The maximal clique implementation in Cliquer (#888) does not perform as well as the one currently in igraph, so the pull request doesn't add it (it's only for non-maximal cliques and weighted cliques).
I'm okay with exposing this and pull request #824 does not seem to affect it. (However, we cannot merge #824 until it has some tests anyway, so if you feel like doing this at some point, just fork from master and ignore #824).
@ntamas Is it going to be sufficient to rename igraph_i_maximal_cliques to igraph_maximal_cliques_callback and make it public (and also igraph_i_maximal_clique_func_t)? Or is there any reason why it would be better to keep it internal and add a public wrapper instead?
Renaming is okay I think. I believe the only reason for them being internal is because we did not want to commit exposing such a functionality in the public API for a long time, but I think it is safe to do so now.