Can't parse quil code with a call to a DEFCIRCUIT with no arguments

This does not parse:

DEFCIRCUIT FOO:
    NOP
FOO

But this is OK:

DEFCIRCUIT FOO a:
    NOP
FOO 1

python -c 'from pyquil._parser.PyQuilListener import run_parser; run_parser("DEFCIRCUIT FOO:\n NOP\nFOO")'

[ecpeterson]

I’d like to add my opinion that I don’t like that the body of a DEFCIRCUIT can reference static qubit literals at all. I’d strongly prefer FOO 1 to account for all of the qubits touched by FOO: the qubit 1, and only qubit 1.

[braised-babbage]

A basic issue is that pyquil has no distinction between gate applications and circuit applications (everything gets parsed into Gate objects), and resolution is done by name via global lookup tables like QUANTUM_GATES.

For example, one easy way to allow for nullary circuit applications in the parser is to change

gate                : modifier* name ( LPAREN param ( COMMA param )* RPAREN )? qubit? ;

to

gate                : modifier* name ( LPAREN param ( COMMA param )* RPAREN )? qubit* ;

in Quil.g4, and correspondingly remove

        if not isinstance(qubits, list) or not qubits:
            raise TypeError("Gate arguments must be a non-empty list")

from Gate.__init__.

But this is a bad idea, because we should be enforcing the non-nullary condition for gate applications.

The way that the quilc parser addresses this is to actually do two passes. The initial parsing produces an “unresolved application” object. A second pass resolves applications to gate applications or circuit applications, depending on the presence of suitable DEFCIRCUIT etc. The resolved objects are tagged with some important information: a pointer to their corresponding gate or circuit definitions. This is an additional level of detail that pyquil does not maintain (in pyquil, a Gate instruction has only a name, with no explicit reference to its corresponding definition).