A specific version of ot-json0 is hardcoded as the basic type

There's currently a bunch of experiments going on around adding presence to ot-json0. One of the things I've come across is that ShareDB currently has a hard dependency on the ot-json0 NPM package, meaning that if we want to replace it with another implementation, we need to also fork the ShareDB package, which is unfortunate.

Here's the code that errors out:

      if (types.map[message.type] !== types.defaultType) {
        err = new ShareDBError(4020, 'Invalid default type');
        return this.emit('error', err);

(lib/client/connection.js:198)

We can either remove that (I did, and it worked), or instead of this line in lib/types.js:

exports.defaultType = require('ot-json0').type;

we might think about letting the user (optionally) supply a default type when configuring ShareDB?

Currently I've just been looking at replacing the base ot-json0 implementation with a type that is almost identical, just with added presence. However, I don't actually know enough about the requirements for a defaultType - does there have to be one? Does it just need to "exist" or to implement a certain interface (beyond the general OT interface)? Could I specify "ot-text" as a default type?

Thanks!

Start by tracing the defaultType assignment in lib/types.js and the validation at lib/client/connection.js:198. Review how ShareDB types are configured and determine the requirements for a replaceable default type. Done should define and document the supported configuration behavior, including whether a custom implementation such as ot-text is valid, with coverage for the validation path.