Initializing an object with Optional (nullable) fields

Say I have a type:

public type Element = {
    name: Text;
    attributes: [(Text, Text)];
    text: Text;
    children: [Element];

I can create instances of this type by specifying values for all of the fields:

var el: Element = {
  name = "my_name";
  attributes = [("my_key0", "my_val0"), ("my_key1", "my_val1")];
  text = "Some text";
  children = [];

But what I really want Element to represent is an object that always has a name, and may have one or more of the rest of the fields, i.e. it may have a couple attributes and some text, but no children as in the above example or it may have no attributes and no text but a few children or some text but no attributes or children. I looked into accomplishing this structure using variants, but I really need an Element to represent any possible permutation, not just a select named few.

I’d like to create instance of Element like the example above but I don’t want to have to specify empty values for the fields I’m not interested in. I figured if I made the fields Optional, they would default to null if not specified, but this doesn’t seem to be how things work.

public type Element = {
    name: Text;
    attributes: ?[(Text, Text)];
    text: ?Text;
    children: ?[Element];
var el: Element = {
    name = "my_title";
    attributes = ?[("my_key0", "my_val0"), ("my_key1", "my_val1")];
    text = ?"Some text";

results in

type error, expression of type
  {attributes : ?[(Text, Text)]; name : Text; text : ?Text}
cannot produce expected type
  {attributes : ?[(Text, Text)]; children : ?[Element]; name : Text; text : ?Text}

How can I create instances of Element by only specifying the fields I’m interested in, and have the non-specified fields default to null?

1 Like

Unfortunately, that’s currently not supported.

Maybe before such a a feature is available, we’ll have “record updates”, where you can create a record from a record, changing just a few fields. Then you could create a defaultElement with all fields null, and create further elements by updating (in the purely functional sense) that value. That might at least alleviate some of the pain.