how to refactor to use middleware?

koa.coffee

# GET /participants
app.get '/participants', parseTimestamp, ->
  options = timestamp: $gte: (+ @query?.start or 0)
  @body = mustache.render table_template, participants: yield participants.find(options)

# GET /participants.json
app.get '/participants.json', parseTimestamp, ->
  options = timestamp: $gte: (+ @query?.start or 0)
  @body = yield participants.find(options)

# GET /participants.csv
app.get '/participants.csv', parseTimestamp, ->
  options = timestamp: $gte: (+ @query?.start or 0)
  @body = yield json2csv data: yield participants.find(options)

koa.js

app.get('/participants', parseTimestamp, function*() {
  var options = {
    timestamp: {
      $gte: +(this.query.start || 0)
    }
  };
  return this.body = mustache.render(table_template, {
    participants: (yield participants.find(options))
  });
});

app.get('/participants.json', parseTimestamp, function*() {
  var options = {
    timestamp: {
      $gte: +(this.query.start || 0)
    }
  };
  return this.body = (yield participants.find(options));
});

app.get('/participants.csv', parseTimestamp, function*() {
  var options = {
    timestamp: {
      $gte: +(this.query.start || 0)
    }
  };
  return this.body = (yield json2csv({
    data: (yield participants.find(options))
  }));
});