Existential Operator / Null Propagation Operator

Igor Baklan io.baklan at gmail.com
Tue Jan 3 13:22:21 UTC 2017


For me ``.?.`` looks more strict. With variations: ``a.?.b.?.c``,
``a.?(args)``, ``a.?[key]``.
Since I would rather look on it like on something similar to [C# extension
method](https://msdn.microsoft.com/en-us//library/bb383977.aspx) (or [Scala
implicit method](
http://docs.scala-lang.org/overviews/core/implicit-classes.html)), and then
in its "full/extended form" it can be

```js
a.$selfOrNullObj().b.$selfOrNullObj().c
```

Where ``.$selfOrNullObj().`` exactly corresponds to ``.?.`` (or in other
words ``.?.`` might be shortcut for ``.$selfOrNullObj().``)

And in terms of [extension method](
https://msdn.microsoft.com/en-us//library/bb383977.aspx) this-argument not
required to be not-null (it is valid case when null passed in place of
this-argument into that static method which is extension method for some
other class).

Then in this case (inspired by that approach) it can be approximately
declared like:

```js
obj.?
```
<==>
```js
obj.$selfOrNullObj()
```
<==>
```js
(obj != null ? obj : {_proto_:null} )
// or (obj != null ? obj : ()=>{}) to be compatible with ``a.?(args)``
```

Then it also could be proposed some variation with fallback to particular
default, like

```js
obj.$selfOrDefault(defObj)
```
<==>
```js
obj.??(defObj)
```
<==>
```js
(obj != null ? obj : defObj )
```

Combining with ``Nil`` approach (some null object which semantically the
same as null, but returns itself for any operation - ``.``, ``[]``, ``()``
; can be easily implemented as ``Proxy`` object for ``()=>{}``), we can
achieve some sort of transitiveness

```js
a.??(Nil).b.c.d.??(null)
```

But in general it fails if for example ``a.b != null`` but ``a.b.c ==
null``, and in this case this construct should work in more advanced way -
like, default value should be used not only for that particular
``.??(Nil).`` operation, but for all consequent ``.`` operations too, until
that default value will not be reset back to ``null`` by trailing
``.??(null)`` operation (which also should replace Nil with null).
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