Graphql dynamic input variables issue

When I tried to add dynamic input for the root query in these examples I can always see an error "Must provide an operation" message.This occurred only in this examples.

> query RootQuery($filter:String) {
>     accounts(filter:$filter){
>        account_number,
>        account_type_code,
>        region
>     }
> }

Can you please add any example for this kind of requirement.