Validation rules. You can create rules what have any type of validation method and error.

Create an rule.

rule
    { field = .age
    , method = (>) 18
    , validWhen = True
    , error = "Age must be greater than 18."
    }

Validate a subject using rules.

validate
    [ rule
        { field = .age
        , method = (>) 18
        , validWhen = True
        , error = "Age must be greater than 18."
        }
    ]
    { age = 18 }