Sometimes you may want to implement a Grails GSP tag that has an optional body. Grails tag closures take one or two arguments, the first a map of the attributes passed to the tag the second a Closure representing the tag body. Even if the tag was not invoked with a body the second argument is not null so doing this does not work:

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def myTag = { attrs, body ->
  if (!body) {
      // render a default body
  } else {
      // render the supplied tag body
  }
}

It turns out that if the tag did not have a body the argument passed to the Closure is always the same constant, so what you can do is:

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import org.codehaus.groovy.grails.web.pages.GroovyPage

def myTag = { attrs, body ->
  if (body.is(GroovyPage.EMPTY_BODY_CLOSURE)) {
      // render a default body
  } else {
      // render the supplied tag body
  }
}