Something like valueOr but for null?

[mnn]

valueOr works well for | undefined fields, but not for | null fields. I didn't see any function to support this, but maybe I overlooked something?

import * as O from 'optics-ts';

interface ZUndef {
  a?: AUndef;
}

interface AUndef {
  b?: BUndef;
}

interface BUndef {
  x: number;
}

const testUndef = () => {
  const defaultA: AUndef = {};
  const defaultB: BUndef = {x: 7};

  const xO = O.optic<ZUndef>().prop('a').valueOr(defaultA).prop('b').valueOr(defaultB).prop('x');

  const data: ZUndef = {};

  const res = O.set(xO)(11)(data);

  console.log('undef', res); // ok: undef { a: { b: { x: 11 } } }
};

testUndef();

// ---

interface ZNull {
  a: ANull | null;
}

interface ANull {
  b: BNull | null;
}

interface BNull {
  x: number;
}

const testNull = () => {

  const defaultA: ANull = {b: null};
  const defaultB: BNull = {x: 7};

  const xO = O.optic<ZNull>().prop('a').valueOr(defaultA).prop('b').valueOr(defaultB).prop('x'); // TS2345: Argument of type 'string' is not assignable to parameter of type 'never'.

  const data: ZNull = {a: null};

  const res = O.set(xO)(11)(data); // TypeError: Cannot read property 'b' of null

  console.log('null', res);
};

testNull();

[akheron]

Yeah, unfortunately there's no support for null yet. At some point I was pondering whether null should be treated the same way as undefined, so that e.g. optional() and valueOr() would consider null a missing value.

[jsit]

What's a good way of working around this? Something with .lens()?

[jsit]

For instance, @mnn, I think you could do this:

  const xO = O.optic<ZNull>()
    .prop('a')
    .lens(
      a => a ?? defaultA,
      (a, update) => update,
    )
    .prop('b')
    .lens(
      b => b ?? defaultB,
      (b, update) => update,
    )
    .prop('x');