Why isn't there a = operator for initializing?

[Geremia]

I understand that the assignment operator = is not a reversible operation, but couldn't, e.g., a = 0 be a shorthand for:

wire zero
a <=> zero

Would this be the proper way to initialize a to 0?

[Geremia]

No, swapping wouldn't be the proper way.

A better way is simply to make more variables, as variables are initialized to 0 by default.
Another way is XOR again (as in this example), as XOR is its own inverse.

Variable values can't be overwritten; ^= is equivalent to initialization if the variable holds value 0.

This is why in loops, arrays are used (for_32.src):

module main( in op(1),out c[4](32),inout a[4](32), inout b[4](32))
if op then
 for $i = 0 to 3 do
    c[$i] ^= (a[$i] + b[$i])
  rof
else
 for $i = 0 to 3 do
    c[$i] ^= (a[$i] - b[$i])
 rof
fi op

It doesn't have: c ^= (a[$i] + b[$i]) or c ^= (a[$i] - b[$i]), ∵ that doesn't mean "overwrite variable c with the RHS" (as it would if ^= were an assignment operator).

[TooMuchDakka]

In SyReC the value of a variable can be modified either with one of the supported assignment operations +=, -=, ^=, ++=, --, ~= or the swap operation <=>.

Variable values can't be overwritten; ^= is equivalent to initialization if the variable holds value 0.

The second part of this statement is correct, applying the ^= assignment operation to a variable that is initialized to 0 is equal to an reversible assignment. Howerver, every assignment and swap operation performs an overwrite of the current variable value, e.g. an assignment of the form a += 2 to the variable a that currently has the value 1 would result in a having the value 3. Additionally, only variables of type wire and out are assumed to be initialized with 0 (see RevLib 2.0.1. specification page 3).

A pattern that one can reuse, and is also applied in the linked example, is the reset of ancillary qubits to their initial state |0⟩ be reapplying all assignment that were performed since its last |0⟩ state. This would then allow for the reuse of the ancillary qubits in another expression or statement.

module main(inout a[2](4))
 wire tmp(4)

 // Set value of ancillary qubits of variable 'tmp'
 tmp ^= a[1];
++= tmp;
// Perform some complex operation
...
// Reset value of ancillary qubits of variable 'tmp'
--= tmp;
tmp ^= a[1]
// Ancillary qubits of variable 'tmp' are now in |0⟩ state again