cl-interpret: Complain, rather than panic, on outlined module

compiler/cl-interpret/src/interpret.rs

@@ -60,10 +60,11 @@ impl Interpret for Static {
 }
 impl Interpret for Module {
     fn interpret(&self, env: &mut Environment) -> IResult<ConValue> {
+        let Self { name, kind } = self;
         // TODO: Enter this module's namespace
-        match &self.kind {
+        match kind {
             ModuleKind::Inline(file) => file.interpret(env),
-            ModuleKind::Outline => todo!("Load and parse external files"),
+            ModuleKind::Outline => Err(Error::Outlined(*name)),
         }
     }
 }

compiler/cl-interpret/src/lib.rs

@@ -580,7 +580,7 @@ pub mod error {
             want: usize,
             got: usize,
         },
-        NullPointer,
+        Outlined(Sym),
     }
 
     impl std::error::Error for Error {}
@@ -620,8 +620,8 @@ pub mod error {
                         if *want == 1 { "" } else { "s" }
                     )
                 }
-                Error::NullPointer => {
-                    write!(f, "Attempted to dereference a null pointer?")
+                Error::Outlined(name) => {
+                    write!(f, "Module {name} specified, but not imported.")
                 }
             }
         }