Added example for error reporting with LALR

examples/README.md

@@ -10,6 +10,7 @@
 
 ### Advanced
 
+- [error\_reporting\_lalr.py](error_reporting_lalr.py) - A demonstration of example-driven error reporting with the LALR parser
 - [python\_parser.py](python_parser.py) - A fully-working Python 2 & 3 parser (but not production ready yet!)
 - [conf.py](conf.py) - Demonstrates the power of LALR's contextual lexer on a toy configuration language
 - [reconstruct\_json.py](reconstruct_json.py) - Demonstrates the experimental text-reconstruction feature

examples/error_reporting_lalr.py

@@ -0,0 +1,81 @@
+#
+# This demonstrates example-driven error reporting with the LALR parser
+#
+
+from lark import Lark, UnexpectedToken
+
+from .json_parser import json_grammar   # Using the grammar from the json_parser example
+
+json_parser = Lark(json_grammar, parser='lalr')
+
+class JsonSyntaxError(SyntaxError):
+    def __str__(self):
+        context, line, column = self.args
+        return '%s at line %s, column %s.\n\n%s' % (self.label, line, column, context)
+
+class JsonMissingValue(JsonSyntaxError):
+    label = 'Missing Value'
+
+class JsonMissingOpening(JsonSyntaxError):
+    label = 'Missing Opening'
+
+class JsonMissingClosing(JsonSyntaxError):
+    label = 'Missing Closing'
+
+class JsonMissingComma(JsonSyntaxError):
+    label = 'Missing Comma'
+
+class JsonTrailingComma(JsonSyntaxError):
+    label = 'Trailing Comma'
+
+
+def parse(json_text):
+    try:
+        j = json_parser.parse(json_text)
+    except UnexpectedToken as ut:
+        exc_class = ut.match_examples(json_parser.parse, {
+            JsonMissingValue: ['{"foo": }'],
+            JsonMissingOpening: ['{"foo": ]}',
+                                 '{"foor": }}'],
+            JsonMissingClosing: ['{"foo": [}',
+                                 '{',
+                                 '{"a": 1',
+                                 '[1'],
+            JsonMissingComma: ['[1 2]',
+                               '[false 1]',
+                               '["b" 1]',
+                               '{"a":true 1:4}',
+                               '{"a":1 1:4}',
+                               '{"a":"b" 1:4}'],
+            JsonTrailingComma: ['[,]',
+                                '[1,]',
+                                '[1,2,]',
+                                '{"foo":1,}',
+                                '{"foo":false,"bar":true,}']
+        })
+        if not exc_class:
+            raise
+        raise exc_class(ut.get_context(json_text), ut.line, ut.column)
+
+
+def test():
+    try:
+        parse('{"key":')
+    except JsonMissingValue:
+        pass
+
+    try:
+        parse('{"key": "value"')
+    except JsonMissingClosing:
+        pass
+
+    try:
+        parse('{"key": ] ')
+    except JsonMissingOpening:
+        pass
+
+
+if __name__ == '__main__':
+    test()
+
+