funception.js

// Accepts a map function, return a push-filter function.
function mapToPush(map) {
  return function (emit) {
    return function (err, item) {
      if (item === undefined) return emit(err);
      emit(null, map(item));
    };
  };
}

// Accepts a map function, returns a pull-filter function
function mapToPull(map) {
  return function (read) {
    return function (close, callback) {
      if (close) return read(close);
      read(null, function (err, item) {
        if (item === undefined) return callback(err);
        callback(null, map(item));
      });
    };
  };
}