From b0e81b05d08b772534763b03107e7663cd79a932 Mon Sep 17 00:00:00 2001
From: Alex van de Sandt <alex@avandesa.dev>
Date: Thu, 18 Jul 2024 16:17:49 -0400
Subject: [PATCH] Handle undefined object fields and non-object derefs

---
 src/interpreter.rs | 48 ++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 44 insertions(+), 4 deletions(-)

diff --git a/src/interpreter.rs b/src/interpreter.rs
index fe99fcb..ba09672 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -209,10 +209,13 @@ impl<'a> Interpreter<'a> {
                 callable.call(self, evaluated_args)
             }
             Expr::Dereference { object, name } => {
-                let Some(instance) = self.interpret_expr(object, env)?.as_instance() else {
-                    todo!("deref target isn't class instance")
-                };
-                let value = instance.get(name).expect("field not defined");
+                let val = self.interpret_expr(object, env)?;
+                let instance = val
+                    .as_instance()
+                    .ok_or_else(|| RuntimeError::deref_non_instance(&val, object.span()))?;
+                let value = instance
+                    .get(name)
+                    .ok_or_else(|| RuntimeError::undefined_field(object.span(), name))?;
                 Ok(value)
             }
         }
@@ -407,6 +410,20 @@ pub enum RuntimeError {
         #[label("this is a `{actual_type}`, not an object")]
         non_instance: SourceSpan,
     },
+    #[error("Only objects have fields")]
+    DerefNonClassInstance {
+        actual_type: &'static str,
+        #[label("this is a `{actual_type}`, not an object")]
+        non_instance: SourceSpan,
+    },
+    #[error("Field `{field_name}` not defined on object")]
+    UndefinedField {
+        field_name: String,
+        #[label("this object...")]
+        object: SourceSpan,
+        #[label("...does not have this field or method defined")]
+        target: SourceSpan,
+    },
 
     #[error(transparent)]
     IoError(#[from] std::io::Error),
@@ -457,6 +474,21 @@ impl RuntimeError {
             non_instance: value_span.into(),
         }
     }
+
+    fn deref_non_instance(non_instance: &Value, value_span: Span) -> Self {
+        Self::DerefNonClassInstance {
+            actual_type: non_instance.type_str(),
+            non_instance: value_span.into(),
+        }
+    }
+
+    fn undefined_field(object_span: Span, name: &Spanned<String>) -> Self {
+        Self::UndefinedField {
+            field_name: name.as_ref().clone(),
+            object: object_span.into(),
+            target: name.span().into(),
+        }
+    }
 }
 
 #[cfg(test)]
@@ -792,6 +824,14 @@ mod test {
         );
     }
 
+    #[test]
+    fn errs_on_non_instance_deref() {
+        assert_matches!(
+            interpret_expr_to_err("1.field"),
+            RuntimeError::DerefNonClassInstance { .. }
+        )
+    }
+
     #[test]
     fn errs_on_illegal_arithmetic() {
         for input in [
-- 
2.51.2