feat(tazjin/rlox): Implement early return from functions r/2108

In the book this is implemented via exceptions as control flow, and
I'm sticking somewhat closely to that by doing it via an error
variant.

Change-Id: I9c7b84d6bb28265ab94021ea681df84f16a53da2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2395
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

1 files changed, 12 insertions, 0 deletions

diff --git a/users/tazjin/rlox/src/errors.rs b/users/tazjin/rlox/src/errors.rs
index 4f173d0160..875f268ca6 100644
--- a/users/tazjin/rlox/src/errors.rs
+++ b/users/tazjin/rlox/src/errors.rs
@@ -1,3 +1,5 @@
+use crate::interpreter::Value;
+
 #[derive(Debug)]
 pub enum ErrorKind {
     UnexpectedChar(char),
@@ -12,6 +14,16 @@ pub enum ErrorKind {
     InternalError(String),
     InvalidAssignmentTarget(String),
     RuntimeError(String),
+
+    // This variant is not an error, rather it is used for
+    // short-circuiting out of a function body that hits a `return`
+    // statement.
+    //
+    // It's implemented this way because in the original book the
+    // author uses exceptions for control flow, and this is the
+    // closest equivalent that I had available without diverging too
+    // much.
+    FunctionReturn(Value),
 }
 
 #[derive(Debug)]