Implement q()

[dgkf]

The current implementation sorely lacks an obvious way to quit. Although you can break out of the repl with Ctrl+D, anyone who's booting up a repl for the first time is likely to use q().

To implement a quit functionality, we need to first implement a new signalling condition to signal a termination. This should be added to the possible R Conditions here:

R/src/lang.rs

Lines 34 to 37 in a902965

	pub enum RSignal {
	Condition(Cond),
	Error(RError),
	}

Next we need to implement a primitive q function. Primitives are callable symbols, and the dispatch to primitive internal calls is handled in this implementation of the Callable trait:

R/src/builtins.rs

Lines 897 to 905 in a902965

	impl Callable for String {
	fn call(&self, args: ExprList, env: &mut Environment) -> EvalResult {
	if let Some(f) = primitive(self) {
	return f(args, env);
	}
	(env.get(self.clone())?).call(args, env)
	}
	}

Which then calls into the primitive function to try to find an appropriate primitive internal call if one exists:

R/src/builtins.rs

Lines 771 to 777 in a902965

	pub fn primitive(name: &str) -> Option<Box<dyn Fn(ExprList, &mut Environment) -> EvalResult>> {
	match name {
	"c" => Some(Box::new(primitive_c)),
	"list" => Some(Box::new(primitive_list)),
	_ => None,
	}
	}

A new primitive callback for q() would need to be added. For now, I wouldn't worry about even handling any arguments. It should just return the new RSignal to terminate the session.

And finally, the last step would to respond to this termination signal in the REPL handler:

R/src/r_repl/repl.rs

Lines 54 to 58 in a902965

	let res = global_env.eval(expr);
	match res {
	Ok(val) => println!("{}", val),
	Err(e) => println!("{}", e),
	}