[Bug] Inheritance Handling in Walker Causes Error with Non-Default Arguments

[musab-mah-7]

Describe the bug

When using inheritance in Jac-Lang, the walker fails when non-default arguments are used after default arguments in the child walker. This is similar to the issue in Python where non-default arguments follow default arguments in data classes.

The error encountered is:

File "/home/musab/miniconda3/lib/python3.12/dataclasses.py", line 585, in _init_fn raise TypeError(f'non-default argument {f.name!r} ' TypeError: non-default argument 'new_name' follows default argument

To Reproduce

1. Define nodes A and B with a name attribute.
2. Create a walker visit_b_by_name that has a default argument create_b_if_not_found.
3. Extend the walker using inheritance in get_b_by_name and update_b_by_name.
4. Trigger the inheritance logic with a root node connected to A and B.
5. Observe the error when trying to create the child walker with non-default arguments after default ones.

Here is the example code that causes the issue:

node A {
    has name: string;
}

node B {
    has name: string;
}

walker visit_b_by_name {
    has name: string;
    has create_b_if_not_found: bool = False;
    can visti_A with `root entry {
        visit [-->];
    }
    can visti_B with A entry {
        visit [-->(`?B)](?name in self.name) else {
            print("B not found with name: ", self.name);
        }
    }
}

walker get_b_by_name:visit_b_by_name {
    can report_B with B entry {
        print("B name: ", here.name);
        report here;
    }
}

walker update_b_by_name:visit_b_by_name {
    has new_name: string;
    can update_B with B entry {
        here.name = self.new_name;
    }
}

with entry:__main__ {
    root ++> A(name="a1") ++> B(name="b1");
    root spawn get_b_by_name(name="b1");
    root spawn update_b_by_name(name="b1", new_name="b2");
    root spawn get_b_by_name(name="b2");
}

Expected Behavior: The inheritance should allow for the proper handling of non-default arguments following default arguments, similar to how Python handles this using the kw_only=True option in data classes.

Possible Solution: In Python, we work around this by making non-default arguments keyword-only using kw_only=True in dataclass. Jac-Lang might need a similar mechanism to enforce keyword-only arguments for inherited walkers.

Workaround (if any): Currently, there doesn't appear to be a workaround within Jac-Lang itself, aside from manually reorganizing argument orders or avoiding inheritance.

Additional Context: The issue also mirrors how Python’s dataclass behaves in inheritance when default and non-default arguments are used. Here’s a similar mapping in Python:

from dataclasses import dataclass

@dataclass
class A:
    var1: int
    var2: str = "hello"

@dataclass(kw_only=True)
class B(A):
    var3: int

# Creating an instance
b_instance = B(var1=10, var3=20)

The solution in Python is to use kw_only=True in the dataclass. It would be helpful to have a similar functionality in Jac-Lang to manage inheritance behavior when mixing default and non-default arguments.

[marsninja]

Hi @musab-mah-7, I dont think the right solution is the kw_only pattern in the lower example, upon investigation it appears a recommended approach to achieve teh desired behavior is

from dataclasses import dataclass

@dataclass
class A:
    var1: int
    var2: str = "hello"  # Optional field with default

@dataclass
class B(A):
    var3: int
    var2: str = "hello"  # Must be redefined to keep the correct order

# Creating an instance
b_instance = B(var1=10, var3=20)

print(b_instance)

Check to see if this works in Jac.

Also we can automate this semantic in our compiler as an improvement to pythons dataclasses, should be somewhat straightforward in the py-ast-gen pass using some lookups in the symbol table. @kugesan1105 what do you think? This would be a nice feature.