It looks like this:

import grails.converters.*
class BookController {
def books
def list = {
this.books = Book.list()
withFormat {
html bookList:books
js { render “alert(’hello’)” }
xml { render books as XML }
}
}
}

Read more about it if you like in the Grails reference docu: http://grails.org/doc/1.0.x/guide/6.%20The%20Web%20Layer.html#6.8%20Content%20Negotiation