Create "unless" statement as a binding from user defined script

Question

I have a task in which I need to implement a processor for user script where in one part of the script unless appears and it needs to behave as the opposite of if statement. Is there a way to create a binding which is going to behave like that. I am new to Groovy so maybe my explanation is not so clear but my code can tell a little bit more about my problem. Replacing unless with if statement works perfectly fine but I need an idea for making unless.

static List<String> filterSitesByUserScript(String userScript, List<String> sites) {

  //properties
  List<String> rememberedSites = new ArrayList<String>()

  //binding
  def binding = new Binding()
  binding['allSites'] = sites
  binding['rememberedSites'] = rememberedSites
  binding['download'] = {String site ->
      new URL(site).getText()
  }
  //binding['unless'] = {statement -> statement == !statement}
  binding['siteTalksAboutGroovy'] = { content -> content.contains("groovy") || content.contains("Groovy") }
  binding['remember'] = { String site -> rememberedSites.add(site)}

  //groovy shell
  GroovyShell shell = new GroovyShell(binding)
  shell.evaluate(userScript)

  return rememberedSites
}

//A test user script input.
String userInput = '''
   for(site in allSites) {
       def content = download site
       unless (siteTalksAboutGroovy(content)) {
           remember site
       }
   }
   return rememberedSites
'''

//Calling the filtering method on a list of sites.
sites = ["http://groovy.cz", "http://gpars.org", "http://groovy-lang.org/", "http://infoq.com", "http://oracle.com", "http://ibm.com"]
def result = filterSitesByUserScript(userInput, sites)
result.each {
    println 'No groovy mention at ' + it
}
assert result.size() > 0 && result.size() < sites.size
println 'ok'

Answer 1

If you want to execute following code:

unless (siteTalksAboutGroovy(content)) {
    remember site
}

you can create a binding unless that stores a closure with two parameters:

binding['unless'] = { test, block -> if (!test) block() }

This closure executes block() only if the first parameter test is false. In case of your example running the code with unless closure would produce following output:

No groovy mention at http://infoq.com
No groovy mention at http://oracle.com
No groovy mention at http://ibm.com