Only raise RuntimeErrors from the Interpreter

[faultyserver]

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With the upgrade to Crystal 0.24.1 (see #53), expect_raises now requires an exception type to be specified by the callers. Out of haste to get 0.3.0 out, I've just put the general Exception class in for most cases where no argument was given. However, this is not a good practice, and the expected exceptions should be better restricted to more relevant types.

I think this improvement has two components: First, find all instances of expect_raises(Exception) in the spec suite and change them to more appropriate types (e.g., ParseError for errors raised by the parser, SyntaxError for errors from the Lexer, etc.). I don't think there are many of these.

Most instances probably won't be resolvable by the above fix, so the second part of this improvement would be to find all instances of raise in the Interpreter source code and turn them into appropriate RuntimeErrors. Not only does this help improve the spec, it also means that users can catch these errors in their programs, which is a good goal.

Adding a __raise_runtime_error helper method to util.cr also seems like a good idea to help mitigate the risk of this issue coming up again in the future.

Here's an example of how one of these improvements could be made. Line 18 here raises a native exception when re-assigning the value of a Constant:

myst/src/myst/interpreter/nodes/simple_assign.cr

Lines 16 to 21 in 4065100

	when Const
	if current_scope.has_key?(target.name)
	raise "Re-assignment to constant value #{target.name}."
	else
	current_scope.assign(target.name, value)
	end

.

Instead of a native exception being raised, I think this should raise a RuntimeError with the same message. Assuming the __raise_runtime_error method from above is implemented, it might look something like this:

# ...
when Const 
  if current_scope.has_key?(target.name) 
    __raise_runtime_error("Re-assignment to constant value #{target.name}.")
  else 
    current_scope.assign(target.name, value) 
  end
# ...

Then, in the specs, there's this test:

myst/spec/interpreter/nodes/simple_assign_spec.cr

Lines 29 to 35 in 30be440

	it "does not allow re-assignment to constants" do
	error = expect_raises do
	parse_and_interpret %q(
	THING = 1
	THING = 2
	)
	end

that should change to expect a RuntimeError instead of a normal Exception.

Hopefully that's enough direction to get started. As always, feel free to ask any questions either here or in the discord server. I don't think this issue requires too much knowledge of the interpreter, but it's hard for me to tell.