Improved Earley error on EOF (Issue #457)

lark/exceptions.py

@@ -13,6 +13,14 @@ class ParseError(LarkError):
 class LexError(LarkError):
     pass
 
+class UnexpectedEOF(ParseError):
+    def __init__(self, expected):
+        self.expected = expected
+
+        message = ("Unexpected end-of-input. Expected one of: \n\t* %s\n" % '\n\t* '.join(x.name for x in self.expected))
+        super(UnexpectedEOF, self).__init__(message)
+
+
 class UnexpectedInput(LarkError):
     pos_in_stream = None
 

lark/parsers/earley.py

@@ -14,7 +14,7 @@ import logging
 from collections import deque
 
 from ..visitors import Transformer_InPlace, v_args
-from ..exceptions import ParseError, UnexpectedToken
+from ..exceptions import UnexpectedEOF, UnexpectedToken
 from .grammar_analysis import GrammarAnalyzer
 from ..grammar import NonTerminal
 from .earley_common import Item, TransitiveItem
@@ -270,6 +270,7 @@ class Parser:
 
         ## Column is now the final column in the parse.
         assert i == len(columns)-1
+        return to_scan
 
     def parse(self, stream, start):
         assert start, start
@@ -288,7 +289,7 @@ class Parser:
             else:
                 columns[0].add(item)
 
-        self._parse(stream, columns, to_scan, start_symbol)
+        to_scan = self._parse(stream, columns, to_scan, start_symbol)
 
         # If the parse was successful, the start
         # symbol should have been completed in the last step of the Earley cycle, and will be in
@@ -306,8 +307,7 @@ class Parser:
 
         if not solutions:
             expected_tokens = [t.expect for t in to_scan]
-            # raise ParseError('Incomplete parse: Could not find a solution to input')
-            raise ParseError('Unexpected end of input! Expecting a terminal of: %s' % expected_tokens)
+            raise UnexpectedEOF(expected_tokens)
         elif len(solutions) > 1:
             assert False, 'Earley should not generate multiple start symbol items!'
 

lark/parsers/xearley.py

@@ -146,4 +146,5 @@ class Parser(BaseParser):
         self.predict_and_complete(i, to_scan, columns, transitives)
 
         ## Column is now the final column in the parse.
-        assert i == len(columns)-1
+        assert i == len(columns)-1
+        return to_scan