2 files changed, 49 insertions, 25 deletions

diff --git a/tvix/eval/src/vm/generators.rs b/tvix/eval/src/vm/generators.rs
index bbf79f47c05d..f2ce73a0c742 100644
--- a/tvix/eval/src/vm/generators.rs
+++ b/tvix/eval/src/vm/generators.rs
@@ -763,9 +763,13 @@ pub(crate) async fn request_span(co: &GenCo) -> LightSpan {
     }
 }
 
-pub(crate) async fn request_to_json(co: &GenCo, value: Value) -> serde_json::Value {
+pub(crate) async fn request_to_json(
+    co: &GenCo,
+    value: Value,
+) -> Result<serde_json::Value, CatchableErrorKind> {
     match co.yield_(VMRequest::ToJson(value)).await {
-        VMResponse::Value(Value::Json(json)) => json,
+        VMResponse::Value(Value::Json(json)) => Ok(json),
+        VMResponse::Value(Value::Catchable(cek)) => Err(cek),
         msg => panic!(
             "Tvix bug: VM responded with incorrect generator message: {}",
             msg
diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
index 057a47bc008c..d9a8a2c411df 100644
--- a/tvix/eval/src/vm/mod.rs
+++ b/tvix/eval/src/vm/mod.rs
@@ -646,7 +646,7 @@ impl<'o> VM<'o> {
                 // exactly.
                 OpCode::OpAssertBool => {
                     let val = self.stack_peek(0);
-                    if !val.is_bool() {
+                    if !val.is_catchable() && !val.is_bool() {
                         return frame.error(
                             self,
                             ErrorKind::TypeError {
@@ -732,17 +732,17 @@ impl<'o> VM<'o> {
                 }
 
                 OpCode::OpConcat => {
-                    let rhs = self
-                        .stack_pop()
-                        .to_list()
-                        .with_span(&frame, self)?
-                        .into_inner();
-                    let lhs = self
-                        .stack_pop()
-                        .to_list()
-                        .with_span(&frame, self)?
-                        .into_inner();
-                    self.stack.push(Value::List(NixList::from(lhs + rhs)))
+                    // right hand side, left hand side
+                    match (self.stack_pop(), self.stack_pop()) {
+                        (Value::Catchable(cek), _) | (_, Value::Catchable(cek)) => {
+                            self.stack.push(Value::Catchable(cek));
+                        }
+                        (rhs, lhs) => {
+                            let rhs = rhs.to_list().with_span(&frame, self)?.into_inner();
+                            let lhs = lhs.to_list().with_span(&frame, self)?.into_inner();
+                            self.stack.push(Value::List(NixList::from(lhs + rhs)))
+                        }
+                    }
                 }
 
                 OpCode::OpResolveWith => {
@@ -797,7 +797,12 @@ impl<'o> VM<'o> {
                         "OpValidateClosedFormals called within the frame of a lambda without formals",
                     );
 
-                    let args = self.stack_peek(0).to_attrs().with_span(&frame, self)?;
+                    let peeked = self.stack_peek(0);
+                    if peeked.is_catchable() {
+                        continue;
+                    }
+
+                    let args = peeked.to_attrs().with_span(&frame, self)?;
                     for arg in args.keys() {
                         if !formals.contains(arg) {
                             return frame.error(
@@ -812,17 +817,24 @@ impl<'o> VM<'o> {
                 }
 
                 OpCode::OpAdd => {
-                    let b = self.stack_pop();
-                    let a = self.stack_pop();
-
-                    let gen_span = frame.current_light_span();
-                    self.push_call_frame(span, frame);
+                    match (self.stack_pop(), self.stack_pop()) {
+                        (Value::Catchable(cek), _) | (_, Value::Catchable(cek)) => {
+                            self.stack.push(Value::Catchable(cek));
+                        }
 
-                    // OpAdd can add not just numbers, but also string-like
-                    // things, which requires more VM logic. This operation is
-                    // evaluated in a generator frame.
-                    self.enqueue_generator("add_values", gen_span, |co| add_values(co, a, b));
-                    return Ok(false);
+                        (b, a) => {
+                            let gen_span = frame.current_light_span();
+                            self.push_call_frame(span, frame);
+
+                            // OpAdd can add not just numbers, but also string-like
+                            // things, which requires more VM logic. This operation is
+                            // evaluated in a generator frame.
+                            self.enqueue_generator("add_values", gen_span, |co| {
+                                add_values(co, a, b)
+                            });
+                            return Ok(false);
+                        }
+                    }
                 }
 
                 OpCode::OpSub => {
@@ -1204,6 +1216,10 @@ async fn resolve_with(
         // TODO(tazjin): is this branch still live with the current with-thunking?
         let with = fetch_forced_with(&co, with_stack_idx).await;
 
+        if with.is_catchable() {
+            return Ok(with);
+        }
+
         match with.to_attrs()?.select(&ident) {
             None => continue,
             Some(val) => return Ok(val.clone()),
@@ -1213,6 +1229,10 @@ async fn resolve_with(
     for upvalue_with_idx in (0..upvalue_with_len).rev() {
         let with = fetch_captured_with(&co, upvalue_with_idx).await;
 
+        if with.is_catchable() {
+            return Ok(with);
+        }
+
         match with.to_attrs()?.select(&ident) {
             None => continue,
             Some(val) => return Ok(val.clone()),