Avoid using `__repr__` for IR serialization implementation

[josh146]

Currently, the intermediate representation uses class composition; there is a container class QasmProgram that stores the parsed program; individual statements are then represented via a nested list of constituent classes (e.g., ArithmeticOperation, ClassicalRegister, Gate, etc.)

This is a nice approach; it preserves the tree-like structure of the program, and provides a convenient method of serialization; simply traverse the IR, and serialize each constituent class/node.

At the moment, each class in the IR computes its serialization via the __repr__ method:

class QuantumRegister(Declaration):
    def __init__(self, name, size):
        super().__init__(decl_type=Declarations.QREG, id_=name, size=size)

    def __repr__(self):
        return f"qreg {self.kwargs['id_']}[{self.kwargs['size']}];"

This was convenient when prototyping, as calling __repr__ is naturally recursive; any IR object appearing in the string returned by __repr__ will itself be serialized.

However, we should probably update this, as this is not ideal; just doing >>> qasm_program on an interactive prompt should not start the serialization process.

Instead, we should do the following:

• Rename the __repr__ method in all IR class to either __str__ or serialize()

  • Renaming it to __str__ allows us to call str(qasm_program) to serialize the program, and avoids us needing to change the logic anywhere --- I'm not sure if this is a good thing or a bad thing.

  def __str__(self):
      return f"qreg {self.kwargs['id_']}[{self.kwargs['size']}];"

  • Renaming it to serialize() allows us to pass arguments later if needed. However, it will need us to update the logic; the returned string will need to manually call serialize on all child objects:

  def serialize(self, *args, **kwargs):
      return f"qreg {self.kwargs['id_'].serialize(*args, **kwargs)}[{self.kwargs['size']}];"

  (Note: the above example is pseudocode just for demonstration; quantum registers do not include nested IR objects).

• Create new __repr__ methods, that return reasonable strings and does not do any recursive serialization:

  def __str__(self):
      return f"<QuantumRegister: name={self.id_}, size={self.size}>"

  That is, the returned string should only itself contain precomputed strings --- it should not result in any additional serialization occurring, either implicitly or explicitly.