Improved load_grammar's error messages, and added tests

lark/load_grammar.py

@@ -789,6 +789,20 @@ def _find_used_symbols(tree):
               for t in x.scan_values(lambda t: t.type in ('RULE', 'TERMINAL'))}
 
 class GrammarLoader:
+    ERRORS = {
+        'Unclosed parenthesis': ['a: (\n'],
+        'Umatched closing parenthesis': ['a: )\n', 'a: [)\n', 'a: (]\n'],
+        'Expecting rule or terminal definition (missing colon)': ['a\n', 'A\n', 'a->\n', 'A->\n', 'a A\n'],
+        'Illegal name for rules or terminals': ['Aa:\n'],
+        'Alias expects lowercase name': ['a: -> "a"\n'],
+        'Unexpected colon': ['a::\n', 'a: b:\n', 'a: B:\n', 'a: "a":\n'],
+        'Misplaced operator': ['a: b??', 'a: b(?)', 'a:+\n', 'a:?\n', 'a:*\n', 'a:|*\n'],
+        'Expecting option ("|") or a new rule or terminal definition': ['a:a\n()\n'],
+        'Terminal names cannot contain dots': ['A.B\n'],
+        '%import expects a name': ['%import "a"\n'],
+        '%ignore expects a value': ['%ignore %import\n'],
+    }
+
     def __init__(self, re_module):
         terminals = [TerminalDef(name, PatternRE(value)) for name, value in TERMINALS.items()]
 
@@ -814,19 +828,9 @@ class GrammarLoader:
                                (e.line, e.column, grammar_name, context))
         except UnexpectedToken as e:
             context = e.get_context(grammar_text)
-            error = e.match_examples(self.parser.parse, {
-                'Unclosed parenthesis': ['a: (\n'],
-                'Umatched closing parenthesis': ['a: )\n', 'a: [)\n', 'a: (]\n'],
-                'Expecting rule or terminal definition (missing colon)': ['a\n', 'a->\n', 'A->\n', 'a A\n'],
-                'Alias expects lowercase name': ['a: -> "a"\n'],
-                'Unexpected colon': ['a::\n', 'a: b:\n', 'a: B:\n', 'a: "a":\n'],
-                'Misplaced operator': ['a: b??', 'a: b(?)', 'a:+\n', 'a:?\n', 'a:*\n', 'a:|*\n'],
-                'Expecting option ("|") or a new rule or terminal definition': ['a:a\n()\n'],
-                '%import expects a name': ['%import "a"\n'],
-                '%ignore expects a value': ['%ignore %import\n'],
-            })
+            error = e.match_examples(self.parser.parse, self.ERRORS, use_accepts=True)
             if error:
-                raise GrammarError("%s at line %s column %s\n\n%s" % (error, e.line, e.column, context))
+                raise GrammarError("%s, at line %s column %s\n\n%s" % (error, e.line, e.column, context))
             elif 'STRING' in e.expected:
                 raise GrammarError("Expecting a value at line %s column %s\n\n%s" % (e.line, e.column, context))
             raise

lark/parsers/grammar_analysis.py

@@ -138,7 +138,7 @@ class GrammarAnalyzer(object):
         for r in rules:
             for sym in r.expansion:
                 if not (sym.is_term or sym in self.rules_by_origin):
-                    raise GrammarError("Using an undefined rule: %s" % sym) # TODO test validation
+                    raise GrammarError("Using an undefined rule: %s" % sym)
 
         self.start_states = {start: self.expand_rule(root_rule.origin)
                              for start, root_rule in root_rules.items()}

tests/__main__.py

@@ -7,6 +7,7 @@ from lark import logger
 from .test_trees import TestTrees
 from .test_tools import TestStandalone
 from .test_cache import TestCache
+from .test_grammar import TestGrammar
 from .test_reconstructor import TestReconstructor
 
 try:

tests/test_grammar.py

@@ -0,0 +1,31 @@
+from __future__ import absolute_import
+
+import sys
+from unittest import TestCase, main
+
+from lark import Lark
+from lark.load_grammar import GrammarLoader, GrammarError
+
+
+class TestGrammar(TestCase):
+    def setUp(self):
+        pass
+
+    def test_errors(self):
+        for msg, examples in GrammarLoader.ERRORS.items():
+            for example in examples:
+                try:
+                    p = Lark(example)
+                except GrammarError as e:
+                    assert msg in str(e)
+                else:
+                    assert False, "example did not raise an error"
+
+
+
+
+if __name__ == '__main__':
+    main()
+
+
+