3 files changed, 33 insertions, 3 deletions
diff --git a/bytecode/src/bytecode.rs b/bytecode/src/bytecode.rs
index 8c9324b..73c8f53 100644
--- a/bytecode/src/bytecode.rs
+++ b/bytecode/src/bytecode.rs
@@ -58,6 +58,8 @@ pub enum Op {
     Cons(Local, Arg, Arg),
     // Length of a list.
     Len(Local, Arg),
+    // Checks that a list has length 1, and gets its first element.
+    AssertSingleton(Local, Arg),
 
     // Locals
     // ======
@@ -261,6 +263,29 @@ impl Interpreter {
         Ok(())
     }
 
+    fn assert_singleton(&mut self, dest: Local, arg: Arg) -> Result<(), String> {
+        let a = self.read_arg(arg);
+        if a == Value::NIL {
+            return Err(String::from(
+                "wrong number of arguments passed to continuation (0)",
+            ));
+        }
+        let l = self.read_arg(arg).to_pointer()?;
+        let typecode = self.heap.peek(l, 0)?;
+        if typecode != Value::from_int(2) {
+            return Err(String::from("not a list"));
+        }
+        let cdr = self.heap.peek(l, 2)?;
+        if cdr != Value::NIL {
+            return Err(String::from(
+                "wrong number of arguments passed to continuation",
+            ));
+        }
+        let car = self.heap.peek(l, 1)?;
+        self.set_arg(dest, car);
+        Ok(())
+    }
+
     fn mov(&mut self, dest: Local, src: Arg) {
         self.set_arg(dest, self.read_arg(src));
     }
@@ -334,6 +359,7 @@ impl Interpreter {
                 Op::PokeByte(word, ptr, offset) => self.poke_byte(word, ptr, offset)?,
                 Op::Cons(dest, x, y) => self.cons(dest, x, y)?,
                 Op::Len(dest, l) => self.len(dest, l)?,
+                Op::AssertSingleton(dest, l) => self.assert_singleton(dest, l)?,
                 Op::Mov(dest, src) => self.mov(dest, src),
                 Op::Jmp(addr) => self.jmp(&mut ip, addr)?,
                 Op::JmpIf(cond, addr) => self.jmp_if(&mut ip, cond, addr)?,
diff --git a/bytecode/src/data.rs b/bytecode/src/data.rs
index e14eaa9..4774f2a 100644
--- a/bytecode/src/data.rs
+++ b/bytecode/src/data.rs
@@ -52,7 +52,10 @@ impl Value {
     pub fn to_pointer(self) -> Result<Pointer, String> {
         let Value(stack_representation) = self;
         if !self.is_pointer() {
-            return Err(format!("value {:x} is not a pointer", stack_representation));
+            return Err(format!(
+                "value 0x{:x} is not a pointer",
+                stack_representation
+            ));
         }
         return Ok(Pointer(usize::try_from(stack_representation).unwrap()));
     }
@@ -69,7 +72,7 @@ impl Value {
     pub fn to_int(self) -> Result<i64, String> {
         let Value(stack_representation) = self;
         if !self.is_int() {
-            return Err(format!("value {:x} is not an int", stack_representation));
+            return Err(format!("value 0x{:x} is not an int", stack_representation));
         }
         return Ok(stack_representation as i64 >> 1);
     }
diff --git a/bytecode/src/encoding.rs b/bytecode/src/encoding.rs
index 44fff51..a4e159f 100644
--- a/bytecode/src/encoding.rs
+++ b/bytecode/src/encoding.rs
@@ -119,8 +119,9 @@ fn op_decoding<T: Read>(prog: &mut T) -> Result<Option<Op>, String> {
             read_arg(prog, arg2_const)?,
         ),
         19 => Op::Len(read_local(prog)?, read_arg(prog, arg1_const)?),
+        20 => Op::AssertSingleton(read_local(prog)?, read_arg(prog, arg1_const)?),
         _ => {
-            return Err(String::from("invalid opcode"));
+            return Err(format!("invalid opcode {tag}"));
         }
     };
     Ok(Some(op))