better cast errors

src/std/cast.rs

@@ -5,17 +5,34 @@ struct CastResolver<'a, Ctx: 'a + Context> {
     points: Vec<Point<'a, Ctx, TypelessMentionable<'a, Ctx>>>,
 }
 
+pub enum CastError<'a> {
+    Typeless(TypelessError<'a>),
+    AddressIndexOutOfBounds(usize),
+    AddressPointMismatch {
+        expected: Hash,
+        received: Hash,
+        index: usize,
+    },
+}
+
 impl<'a, Ctx: 'a + Context> Resolver<'a, Ctx> for CastResolver<'a, Ctx>
 where
-    Ctx::LookupError: From<TypelessError<'a>>,
+    Ctx::LookupError: From<CastError<'a>>,
 {
     fn resolve(self: Rc<Self>, address: Address) -> HashResolution<'a, Ctx> {
-        let point = &self
-            .points
-            .get(address.index)
-            .expect("CastResolved received an invalid address.");
+        let point = match self.points.get(address.index) {
+            Some(point) => point,
+            None => {
+                return Ctx::T::pure(Err(CastError::AddressIndexOutOfBounds(address.index).into()));
+            }
+        };
         if point.point != address.point {
-            panic!("CastResolved received an invalid address.")
+            return Ctx::T::pure(Err(CastError::AddressPointMismatch {
+                expected: point.point,
+                received: address.point,
+                index: address.index,
+            }
+            .into()));
         }
         Ctx::T::fmap(
             |resolved| match resolved {
@@ -29,7 +46,7 @@ where
                 }
                 Err(error) => Err(match error {
                     ResolutionError::Lookup(lookup_error) => lookup_error,
-                    ResolutionError::Parse(parse_error) => parse_error.into(),
+                    ResolutionError::Parse(parse_error) => CastError::Typeless(parse_error).into(),
                 }),
             },
             point.resolve(),
@@ -41,7 +58,7 @@ pub type CastResult<'a, Ctx, A> =
     Result<A, <<A as Mentionable<'a, Ctx>>::Fctr as Factory<'a, Ctx>>::ParseError>;
 impl<'a, Ctx: Context> TypelessMentionable<'a, Ctx>
 where
-    Ctx::LookupError: From<TypelessError<'a>>,
+    Ctx::LookupError: From<CastError<'a>>,
 {
     /// .
     ///