Check if element exists in a leaf-list and return error

[kss.confd]

Is it possible to check for an element in a leaf list and return error-message in yang. Please provide an example.

[mvf]

It is a bit unclear what you want to achieve. If you want to:

1. make sure that the leaf-list has at least one element:

   leaf-list data {
      type string;
      min-elements 1;
    }
   

2. make sure that a certain element exists:

   leaf-list data {
     type string;
   }
   must "data = 'myvalue'" {
     error-message "element 'myvalue' must be present";
   }
   

   This can be used event for 1. using simple must "data", the advantage is that it allows you to customize the error message; it might have a bit worse performance though;

3. have a complex constraint on every element in the list - this is possible too, depending on the constraint you may be able to achieve that with a must statement like this:

   leaf-list data {
     type string;
     must "my-xpath-expr";
   }
   

   or you may need to resort to semantic validation. There are examples of this in intro/validation examples (and others), one full chapter of the ConfD user guide is dedicated to this topic too.