wai-quantum: OpenQASM 3 generators, one program for a family of circuits #466

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2026-10-25

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wai-quantum: OpenQASM 3 generators, one program for a family of circuits ...

The reader refused loops, so a family of circuits (a GHZ state on n qubits,
the QFT, an adder) had to be written out once per size. It now reads
generators, the first piece of the specification-and-evaluator workbench,
whose proposers evolve exactly such programs and whose evaluator checks
them across n.

- qasm::parse_with and qasm::parse_lines_with read a program at given
  integer inputs; qasm::inputs lists the inputs it declares. `input int`,
  `input uint`, `const int|uint`, and `for` over [a:b], [a:step:b] (both ends
  included) and {a, b, c}, nested, are unrolled on reading. Indices and sizes
  take integer arithmetic with C's truncating division, `%`, `**`,
  comparisons and `&& || !`. An `if` on integers alone is decided on reading;
  one on bits stays classical control. Unrolled gates keep the lines of
  their loop's body.
- qasm::instantiate writes the circuit at given inputs out as plain
  OpenQASM, which reads back as the same program, gate for gate.
- Bounded: at most 20 million tokens, checked integer arithmetic, and a
  name an integer would shadow (a register, a gate's parameter or qubit, a
  word of the language) is refused with its line. A loop variable may still
  share a gate's name, such as the identity `i`, since an integer is only
  substituted where it is not starting a statement.
- The lexer reads `% < > <= >= && ||`.
- The CLI takes `--input NAME=VALUE` on every command, and `instance`
  prints a generator written out.

A program with no input, const or for reads exactly as before: the
unrolling pass returns its tokens untouched.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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