feat: override of imported grammar rules

Author: igordejanovic

docs/grammar_modularization.md

@@ -44,6 +44,42 @@ tests](https://github.com/igordejanovic/parglare/blob/master/tests/func/import/f
 for an example.
 
 
+## Imported rules override
+
+Grammar rules defined in imported grammar can be overridden by using appropriate
+FQN rule name. For example, if we have following grammars:
+
+file: `first.pg`
+```
+import 'second.pg' as s;
+...
+
+```
+
+file: `second.pg`
+```
+import 'third.pg' as t;
+...
+```
+
+file `third.pg`
+```
+S: A B+;
+B: 'foo' C;
+```
+
+In `first.pg` we could then write:
+```
+S: s.t.S;
+s.t.B: 'bar' C+;
+```
+Which will effectively override rule `B` from `third.pg` (note FQN `s.t.B`).
+
+All grammar rules that used old `B` from `third.pg` will now use `s.t.B` rule
+from `first.pg`. This enables a flexible modification of existing grammars from
+importing grammars just in places where it is needed without having to rewrite
+all the rules that use the rule that needs to be modified.
+
 
 ## Grammar file recognizers
 

parglare/common.py

@@ -21,14 +21,14 @@ class Location:
         end and input_str.
     """
 
-    __slots__ = ['context', 'file_name',
-                 '_line', '_column',
-                 '_line_end', '_column_end']
+    __slots__ = ['start_position', 'end_position', 'input_str', 'file_name',
+                 '_line', '_column', '_line_end', '_column_end']
 
     def __init__(self, context=None, file_name=None):
-
-        self.context = context
-        self.file_name = file_name or context.file_name
+        self.start_position = context.start_position if context else None
+        self.end_position = context.end_position if context else None
+        self.input_str = context.input_str if context else None
+        self.file_name = file_name or context.file_name if context else None
 
         # Evaluate this only when string representation is needed.
         # E.g. during error reporting
@@ -63,40 +63,26 @@ def column_end(self):
         return self._column_end
 
     def evaluate_line_col(self):
-        context = self.context
         self._line, self._column = pos_to_line_col(
-            context.input_str, context.start_position)
+            self.input_str, self.start_position)
 
     def evaluate_line_col_end(self):
-        context = self.context
-        if hasattr(context, 'end_position') \
-                and context.end_position:
+        if self.end_position:
             self._line_end, self._column_end = \
-                pos_to_line_col(context.input_str, context.end_position)
-
-    def __getattr__(self, name):
-        if self.context is not None:
-            return getattr(self.context, name)
-        else:
-            raise AttributeError(name)
+                pos_to_line_col(self.input_str, self.end_position)
 
     def __str__(self):
-        if self.context is None:
-            line, column = None, None
-        else:
-            line, column = self.line, self.column
-        context = self.context
+        line, column = self.line, self.column
         if line is not None:
             return ('{}{}:{}:"{}"'
                     .format(f"{self.file_name}:"
                             if self.file_name else "",
                             line, column,
-                            position_context(context.input_str,
-                                             context.start_position)))
-        elif self.file_name:
+                            position_context(self.input_str,
+                                             self.start_position)))
+        if self.file_name:
             return _a(self.file_name)
-        else:
-            return "<Unknown location>"
+        return "<Unknown location>"
 
     def __repr__(self):
         return str(self)

parglare/glr.py

@@ -511,7 +511,7 @@ def _do_error_recovery(self):
                 successful = self.error_recovery(head, error)
 
             if successful:
-                error.location.context.end_position = head.position
+                error.location.end_position = head.position
                 if debug:
                     a_print("New position is ",
                             pos_to_line_col(input_str, head.position),