Passing Expressions as Expression Parameters

Question

Are there any plans to allow for the passing expressions as expression parameters? I think this would be an incredibly useful feature to add to the expression language.

Philip Snyman · Accepted Answer

Thank you. Yes, I am familiar with what you describe above. I am however referring to the passing of an expression rule as an argument ( rule input ) to another expression rule; not passing the result of an expression rule to another expression rule. 
 I have experimented and I believe I have answered my own question. Here is an explanation if you're interested. It relates to the scenario of another thread we have been discussing ( https://community.appian.com/discussions/f/general/11194/accessing-cdt-field-properties ). So, I have a reusable interface expression that accepts AnyType of rule input where the inputs can be different CDTs. These CDTs do however have common field names, i.e. name, description. I want to pass a CDT specific validation ( expression rule ) as a rule into the interface expression that checks if the value entered for the name field already exists in the database, i.e. through the use of a!queryEntity. This would of course be different for each CDT. So, for example, I would have two expression rules named isCustomerNameUnique(name) and isProductNameUnique(name) . Each of them would check if the name is unique and then return either null if it unique or a validation message indicating the the name is not unique, i.e. "The customer name already exists." or "The product name already exists.". The interface expression would then accept a rule input of AnyType called validation as well as a rule input of AnyType called entity. I could then invoke that expression when I need to validate user input. For example: 
 a!textField(
 label: "Name",
 value: ri!entity.name,
 saveInto: ri!entity.name,
 validations: ri!validation(ri!entity.name)
) 
 Works! Thanks for getting me to think through this carefully! The answer came to me when I explored the concepts detailed here : https://docs.appian.com/suite/help/17.3/Expressions.html#passing-functions-rules-and-data-types-as-arguments 
 What do you think?

gianninol · Answer

You can pass expression rule references via Any Type parameters. For instance, in the below example, we control the transformation function that is used to change a local variable when a button is clicked. When the first button is pressed, the value will have the logarithm with base 5 taken. When the first button is pressed, the value with be taken to the 5th power. This works with any expression rule in addition to functions. 
 Make a button widget rule called myButtonWidget with inputs ri!value and ri!transformFunction with the following code: 
 a!buttonWidget( 
 label: "Transform Value", 
 saveInto: a!save(ri!value, ri!transformRule(ri!value)) 
 ) 
 Then, in an interface, call this rule twice with different transformation functions: 
 load( 
 local!value: 10, 
 a!buttonLayout( 
 primaryButtons: { 
 rule!myButtonWidget(local!value, log(_, 5)), 
 rule!myButtonWidget(local!value, power(_, 5)) 
 } 
 ) 
 )