What to do about 'mode'

[gabejohnson]

The mode property on a table entry bugs me. It's inherited from particular Lisp implementations of readtables.

In the sweet.js reader, the only flag that sets the reader into a particular mode is dispatch. Though that mode could be done away if ideas from #2 or #3 are implemented.

It is also used to communicate to readIdentifier and readNumericLiteral that reading should terminate via entry.mode === 'terminating'. But this is really just a message from one entry to another.

This property could be generalized to pass JSON serializable messages to actions or to the reader.

const someEntry = {
  key: '@',
  message: '{  "maxPrefixLength": 3  }',
  action(stream) { ... }
}

const someOtherEntry = {
  action: function(stream) {
    let char, value = '', table = getCurrentReadtable();
    while (...) {
      char = stream.peek();
      const mapping = table.getMapping(char);
      if (mapping && mapping.message) {
        const msg = JSON.parse(message);
        if (msg.maxPrefixLength) {
          if (value.length + 1 <= msg.maxPrefixLength) return value + stream.readString();
        } else if(msg.terminating) {
          return value;
        }
      }
      value += stream.readString();
    }
    return value;
  }
}

/cc @disnet

[gabejohnson]

Thinking a bit more about this I think we can do away even having a message field. We could just map each key to a function. If the function is called w/ no args It acts like a thunk and returns everything in the entry besides action. Otherwise it returns the result of calling action.

const someEntry = {
  key: '@',
  maxPrefixLength: 3,
  action(stream) { ... }
};

// add extend readtable ...

table.get('@')() // { maxPrefixLength: 3 }
table.get('@')(stream) // runs action