Support `frozen=True` in mlc dataclasses

[junrushao-openai]

When developing IRs, one common request is to have the IR immutable so that debugging experience would be easier. Immutability in IR node directly translates to frozen=True in Python dataclasses (I believe), so I wonder if we could get it natively supported in mlc

[potatomashed]

There are a few dataclass features on our roadmap to implement:

• Support frozen=True;
• Support init=False in dataclass fields;
• Support asdict() and astuple().

Supporting frozen=True is something I'm interested in doing immediately.

[potatomashed]

Regarding frozen=True: In Python's native dataclasses, frozen=True doesn't guarantee list/dict are immutable. Example:

import dataclasses

@dataclasses.dataclass(frozen=True)
class Node:
    lst: list = dataclasses.field(default_factory=list)

node = Node()
node.lst.append(1)  # still mutable
print(node)

In this particular case, we will have to break compatibility with Python as a compiler infra.

I guess what we will do is to

1. introduce mlc.FrozenList and mlc.FrozenDict
2. recursively convert list/dict to FrozenList and FrozenDict if we have frozen=True

Not quite sure if I capture everything needed. Feel free to suggest!

[potatomashed]

Fixed by #54.

Note that as I mentioned previously, there's still mutability even if we set frozen=True in python's native dataclasses, in cases where a field is a container (e.g. list, dict), users can still mutate its content.

import dataclasses

@dataclasses.dataclass(frozen=True)
class Node:
    lst: list = dataclasses.field(default_factory=list)

node = Node()
node.lst.append(1)  # still mutable
print(node)

This is certainly not desirable as an IR infra, so next we are gonna introduce FrozenList and FrozenDict, and automatically convert list/dict to FrozenList/FrozenDict when frozen=True

[junrushao-openai]

Yeah I like the idea replacing mlc.List to mlc.FrozenList. It's a bit too intelligent but should be good UX improvement

[junrushao-openai]

Another idea: we could decide to freeze or unfreeze a list/dict 🤔

[potatomashed]

freeze or unfreeze a list/dict 🤔

Seems a good idea and it's consistent with the API from frozenlist: https://github.com/aio-libs/frozenlist

PR #58