I can see the idea behind allowing map to accept a value, and it's probably useful, but it's also confusing and breaks a long standing contract (the signature of a map function). The reason I'm here is because of this:

class Index {
  lookupValue { ...pretend code... }

  getValueAtTime(time) {
    return this.lookupValue(time);
  }
}

var index = new Index();
var stream = someOtherStream.map(index.getValueForTime);

This returns a stream of undefined.
How long did it take you to see the error there? Took me hours.

Highland.value(val) (there's maybe a better name) would clear this up and save me some sanity.