# How to add a validation to a command?

Define validation checks as pre-conditions inside the command definition.

1. With in the command definition, after input (if any), specify the validations inside a pre block.

   
   
   

    

   
   
   
   
   

   command <command-name>{
   		input(...)
   		pre{
   			condition <conditionName> => <logic> failing "<error-message>"
   		}
   }
   

2. Each pre-condition:

   1. starts with keyword condition
   2. followed by an appropriate name to the condition being checked. For ex: AmountMustBeGreaterThanZero
   3. followed by symbol =>
   4. followed by logic that checks the condition. Use one of the following options:
      1. Call a domain-logic function. [Recommended]
      2. Specify the logic as scala code within back-quotes (only encouraged when the logic is simple)
   5. followed by the error message that needs to be printed when the condition fails, defined in quotes after the keyword failing [Optional]

For example: To ensure the lottery name is unique & the amount given as input is greater than zero:

command create{
		input(lotteryName:String,amount:Int)
		pre{
			condition NewLotteryName => !(lotteryDomainLogicRef.lotteryNameExists(input.lotteryName)) failing "Lottery Name Already Exists !"
			condition AmountMustBeGreaterThanZero => `input.amount > 0`
		}
		...
}

Note:

• Multiple conditions can be composed using and & , or |, implies -> , iff <-> operators

• Also possible to hold some computed properties using let inside the pre block.

• If no error message is specified as part of failing <error-message> condition name becomes the default error message when the condition fails.

• You can use paramaters in the error message (String interpolation) using ${x}notation. For ex:

  ... failing "Lottery Name ${input.lotteryName} Already Exists !"

• In order to call a domain logic function (step 2.4.1), hold the domain-logic as a refernce inside the command set.

  
  

   

  
  
  
  
  

  command-set Lottery {
  	domain-logic-ref lotteryDomainLogicRef:Lottery
  	
  	command create{ ... }
  }