fix(checker): avoid raising on unknown variable

when an unknown variable is referenced, avoid raising an error and only report it with a diagnostic, returning `UnknownType` instead

midas/checker/midas.py

@@ -110,7 +110,7 @@ class MidasTyper(m.Stmt.Visitor[None], m.Expr.Visitor[Type], m.Type.Visitor[Type
             return self._local_variables[name]
         return self.types.get_type(name)
 
-    def get_variable(self, name: str) -> Type:
+    def get_variable(self, location: Location, name: str) -> Type:
         """Get the type of a variable
 
         This function will first look into the current predicate's parameters if
@@ -118,11 +118,9 @@ class MidasTyper(m.Stmt.Visitor[None], m.Expr.Visitor[Type], m.Type.Visitor[Type
         The the variable is looked up in the preamble (i.e. global environment)
 
         Args:
+            location (Location): the location of the variable reference
             name (str): the name of the variable
 
-        Raises:
-            NameError: if the variable cannot be found
-
         Returns:
             Type: the type of the variable
         """
@@ -136,7 +134,8 @@ class MidasTyper(m.Stmt.Visitor[None], m.Expr.Visitor[Type], m.Type.Visitor[Type
         if global_ is not None:
             return global_
 
-        raise NameError(f"Unknown variable '{name}'")
+        self.reporter.error(location, f"Unknown variable '{name}'")
+        return UnknownType()
 
     def resolve(self, stmts: list[m.Stmt]):
         """Process a sequence of statements
@@ -350,7 +349,7 @@ class MidasTyper(m.Stmt.Visitor[None], m.Expr.Visitor[Type], m.Type.Visitor[Type
         return member
 
     def visit_variable_expr(self, expr: m.VariableExpr) -> Type:
-        return self.get_variable(expr.name.lexeme)
+        return self.get_variable(expr.location, expr.name.lexeme)
 
     def visit_grouping_expr(self, expr: m.GroupingExpr) -> Type:
         return expr.expr.accept(self)
@@ -370,7 +369,7 @@ class MidasTyper(m.Stmt.Visitor[None], m.Expr.Visitor[Type], m.Type.Visitor[Type
                 return UnknownType()
 
     def visit_wildcard_expr(self, expr: m.WildcardExpr) -> Type:
-        return self.get_variable("_")
+        return self.get_variable(expr.location, "_")
 
     def visit_named_type(self, type: m.NamedType) -> Type:
         name: str = type.name.lexeme