Introduce constants like None

[namannimmo10]

I think we also need to implement Constant so that we support simple types such as None. Now the question is, how to represent that in ASR? Do we need a Constant node in ASR?

[certik]

We could create a None type, so you could use it like this:

a: NoneType
a = None

But it wouldn't be very useful this way.

The only usage could be with pointers, where None represents nullptr.

What is some usecase that you have in mind?

[namannimmo10]

None is used to define a null value or 'no' value. So, here are some of its common use-cases I notice in the Python codebase:

1. Very often used in function arguments,

   >>> def func(a, b=None):
   ...   if b is None:
   ...     b = []
   ...   b.append(a)
   ...   return b
   ...
   >>>

2. When a variable has no meaningful initial value, we assign None to it.
3. Generally is and is not operators are used in comparing None with other values. I implemented these two in !27, but your comment says something different. What do you have in mind, in regards to handing None elsewhere?

[certik]

Right. 3. is just the syntax if we introduce None. The 1. and 2.are the use cases, and to implement those, we effectively have to introduce a new type system in ASR that allows to define a type like list | NoneType. Possibly some types like list or any instance of a class could have a value of "null", which is represented by None. However, None is also used for things like integers, and for those the new type would be i32 | NoneType and then internally it would have to be represented somehow: either as a special integer value, or using a pointer; and in both cases we would have to insert checking at runtime, so there is a performance hit. So at least initially I would not allow i32 | NoneType until we get more experience with LPython and the performance. The list is currently representing as a 1D array. One can implement None as "unallocated", but it doesn't map exactly when you return an allocatable array, as it can be assign to an unallocatable one. The idiom:

>>> def func(a, b=None):
...   if b is None:
...     b = []
...   b.append(a)
...   return b

can be implemented in ASR using optional arguments, which is what this is. And one can use present(b) to check if the argument b is present.

Anyway, these are the issues. If you can think of some good way forward, then we can discuss it. Otherwise I suggest to wait with this until we implement more of Python, and return back to this in couple of weeks.