# What to write a complex validation?

• Use let to hold computed properties inside pre block

  pre{
  	let participantCount:Int = `domainLogicRef.lottery.participantCount(input.lotteryName)`
  	condition atLeastTwoParticipants => 
                  existingLotteryName -> ` participantCount > 1` failing "The lottery does not have any participant!"
  }
  

• To execute a check only when a pervious condition passes use the condition name with -> operator.

  
  
  

   

  
  
  

  pre{
  	condition existingLotteryName => lottery.lotteryNameExists(input.lotteryName) 
  	condition openToRun => existingLotteryName -> lottery.isOpenToRun(input.lotteryName)
  }
  

• To execute a condition only when an optional input's value exists use keyword present along with -> operator.

  
  
  
  

   

  
  
  
  

  command addParticipant{
  		input(lotteryName:String,participantName:String,email:String?)
  		pre{
  		condition validEmail => present(input.email) -> lottery.validateEmail(input.email.dig)
  		}
  }
  

• Use ! to apply negation inside the condition logic

  
  

   

  
  
  
  

  pre{
  		condition NewLotteryName => !(lottery.lotteryNameExists(input.lotteryName)) failing "Lottery Name Already Exists !"
  			...
  }