title: How to update some (but not all) fields of a record?
body: <p>I have a list named <code>diags</code> of type <code>Linol_lwt.Diagnostic.t list</code>
  I want to update every element of this list and return another one.
  Let's suppose that we have these modules:</p>
  <pre><code>module Diagnostic = struct
    type t =
      { range : Range.t
      ; severity : DiagnosticSeverity.t Json.Nullable_option.t
            [@default None] [@yojson_drop_default ( = )]
      ; code : Jsonrpc.Id.t Json.Nullable_option.t
            [@default None] [@yojson_drop_default ( = )]
      ; source : string Json.Nullable_option.t
            [@default None] [@yojson_drop_default ( = )]
      ; message : string
      ; tags : DiagnosticTag.t list Json.Nullable_option.t
            [@default None] [@yojson_drop_default ( = )]
      ; relatedInformation :
          DiagnosticRelatedInformation.t list Json.Nullable_option.t
            [@default None] [@yojson_drop_default ( = )]
      }
  end
  </code></pre>
  <pre><code>module Range = struct
    type t =
      { start : Position.t
      ; end_ : Position.t
      }
  end
  </code></pre>
  <pre><code>module Position = struct
    type t =
      { line : int
      ; character : int
      }
  end
  
  </code></pre>
  <p>I want to update the field <code>range</code>, so I did this:</p>
  <pre><code>let diags2 = List.map ( fun (x: Linol_lwt.Diagnostic.t) -&gt;
  
                              let ls = x.range.start in
                              let le = x.range.end_ in
                              {
                                Linol_lwt.Diagnostic.range =
                                  {
                                    start = { ls with line = ls.line + 1 };
                                    end_ = { le with line = le.line + 1 }
                                  }
                              }
                     ) diags in
  </code></pre>
  <p>But it says that there is an error with this message:</p>
  <pre><code>Some record fields are undefined: severity code source message tags relatedInformation
  </code></pre>