[BUG] Compiler bug of walrus operator with multiple ands

[YichengDWu]

Bug description

Having multiple assignments in if or while statement results in an error

Steps to reproduce

def main():
  if (a:=True) and (b:=True):
    print(a)
    print(b)
 
/source/prog.mojo:8:10: error: use of uninitialized value 'b'
    print(b)
         ^
/source/prog.mojo:6:21: note: 'b' declared here
  if (a:=True) and (b:=True):
                    ^
mojo: error: failed to run the pass manager

System information

The playground

[soraros]

I don't think this one is a bug. It looks trivial in this particular case, but in general the compiler can't prove that b is always initialised because of short circuiting.

[YichengDWu]

In general, for if (a:=A) and (b:=B), short circuiting means this branch is not taken and thus there's no need to worry about uninitialized references inside the branch. What you said is true for references outside the branch.

[bgreni]

Regardless it's valid Python code so if possible it must work in Mojo as well.

[YichengDWu]

That's a bigger problem. Python would raise a run-time error and mojo is doing some compile-time checks.

Note that the current compiler already know a must be initialized inside the branch, push also b into some kind of list in the pass is an easy and quick fix for this specific bug.

[bgreni]

I meant that exact code runs successfully with no issue (however obviously it fails if you use and or instead). So not only should it be possible, but it is a regression from the expected behaviour in Python.

[YichengDWu]

Yeah it's a serious issue if all valid python code with def should also be valid mojo code.

Mojo is a superset of Python.

[soraros]

In general, for if (a := A) and (b := B), short circuiting means this branch is not taken and thus there's no need to worry about uninitialized references inside the branch. What you said is true for references outside the branch.

I don't think that's correct. In a statement like if e1 and e2, b gets initialised only if e2 gets evaluated. We happens to be in a very special case where b has to be True. But consider this:

fn f(b: Bool) -> Bool:
  return ...

fn g(b: Bool) -> Bool:
  return ...

def test():
  if g((a := A) and f(b := B)):  # f is opaque
    print(a)
    print(b)

We can't guarantee that b is initialised. (Actually, we can if we only have a and.) One should also notice that the constructor of Bool is pretty side effect free. Should we have a more complex type, the unification problem True ~ f(?b) becomes super unsolvable.

Yeah it's a serious issue if all valid python code with def should also be valid mojo code.

Isn't it a good thing that a runtime error got turned into a compile time one?

Anyhow, my point being that we can't support even the slightly more general case until full dict based dynamism.

[YichengDWu]

You changed my argument with evoking g.

Isn't it a good thing that a runtime error got turned into a compile time one?

Good if not buggy.

[soraros]

@YichengDWu So you would agree that if it was if (a := A) or (b := B), it should error out?

[YichengDWu]

It depends on the level of dynamism. if (a := A) and (b := B) should not error on any level.

[soraros]

b is indeed not initialised if a is True, which is not buggy ("good if not buggy"). So I don't understand your argument.

[YichengDWu]

Sorry there was a typo. Corrected:

if (a := A) and (b := B) should not error on any level.