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"No hash" error in offside-rule left recursive grammar
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Daniel Ermak  
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 More options Apr 18 2010, 11:47 am
From: Daniel Ermak <danar...@gmail.com>
Date: Sun, 18 Apr 2010 08:47:48 -0700 (PDT)
Local: Sun, Apr 18 2010 11:47 am
Subject: [LEPL] "No hash" error in offside-rule left recursive grammar
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Hello,

I'm using lepl 4.0 on Python 3.1.2 on Linux. The following grammar
works as expected:

CLine = ContinuedBLineFactory(Token(r'\\'))

expr0 = Token("[A-Za-z_][A-Za-z0-9_]*")

expr1 = Delayed()

call = expr1 & expr0 > List # Deliberately not expr0 & expr1
expr1 += call | expr0

program = expr1 & Eos()
parsed = program.parse("a b c")
print(parsed[0])

This prints:

List
 +- List
 |   +- 'a'
 |   `- 'b'
 `- 'c'

But when I replace the program= line with:

program = (CLine(expr1) & Eos())
program.config.default_line_aware(block_policy=rightmost)

I get this error:

Traceback (most recent call last):
  File "/home/danarmak/workspace/ScalySynth/src/synth/test.py", line
27, in <module>
    parsed = program.parse("a b c")
  File "/usr/lib/python3.1/site-packages/LEPL-4.0-py3.1.egg/lepl/core/
config.py", line 914, in parse
    return self.get_parse()(stream, **kargs)
  File "/usr/lib/python3.1/site-packages/LEPL-4.0-py3.1.egg/lepl/core/
parser.py", line 246, in single
    return next(raw(arg, **kargs))[0]
  File "/usr/lib/python3.1/site-packages/LEPL-4.0-py3.1.egg/lepl/core/
parser.py", line 136, in trampoline
    value = next(value.generator)
  File "/usr/lib/python3.1/site-packages/LEPL-4.0-py3.1.egg/lepl/
stream/filters.py", line 390, in _match
    generator = self.matcher._match(transform.stream)
  File "/usr/lib/python3.1/site-packages/LEPL-4.0-py3.1.egg/lepl/
matchers/memo.py", line 221, in _match
    if key not in self.__caches:
  File "/usr/lib/python3.1/site-packages/LEPL-4.0-py3.1.egg/lepl/
stream/stream.py", line 380, in __hash__
    return hash(self.__line) ^ self.__offset
  File "/usr/lib/python3.1/site-packages/LEPL-4.0-py3.1.egg/lepl/
stream/stream.py", line 561, in __hash__
    return self.source.hash_line(self)
  File "/usr/lib/python3.1/site-packages/LEPL-4.0-py3.1.egg/lepl/
stream/stream.py", line 710, in hash_line
    self.__class__.__name__))
Exception: No hash for [(['Tk9'], 'a')], <bound method Line.location
of [(['Tk9'], 'a')]> (CachingTransformedSource)

The grammar is deliberately left-recursive: I want it to describe left-
associative function calls. If I make it right-recursive (call = expr0
& expr1), then it works with the offside rule, but that's not what I
want. Can I achieve what I want and what does this error mean? (What
does memoization and eliminating left-recursive loops have to do with
making a grammar whitespace-aware?)

Thanks!

Daniel Ermak

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andrew cooke  
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 More options Apr 18 2010, 12:03 pm
From: andrew cooke <and...@acooke.org>
Date: Sun, 18 Apr 2010 09:03:18 -0700 (PDT)
Local: Sun, Apr 18 2010 12:03 pm
Subject: [LEPL] Re: "No hash" error in offside-rule left recursive grammar
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Ah :o(

OK, to answer your last question first, getting left recursion to work
correctly requires hashing (so that we can detect when we process the
same data again and again), while white space parsing is implemented
by modifying the token stream to include flags indicating the start
and end of a line.  Unfortunately, it appears, on first glance, that
the modification process has a bug that breaks hashing.

So, this is a bug I need to fix.

Separate from that, do you need a left recursive grammar to describe
left-associative function calls?  I am pretty sure that the answer has
to be "no", and you really should think about that, because there's a
big penalty (both efficiency and underlying code complexity - hence
this bug) in using left-recursive grammars.

Anyway, I'll look at fixing this ASAP - sorry for the bug and thanks
for the report,
Andrew

On Apr 18, 11:47 am, Daniel Ermak <danar...@gmail.com> wrote:

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Daniel Armak  
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 More options Apr 18 2010, 12:27 pm
From: Daniel Armak <danar...@gmail.com>
Date: Sun, 18 Apr 2010 19:27:30 +0300
Local: Sun, Apr 18 2010 12:27 pm
Subject: Re: [LEPL] Re: "No hash" error in offside-rule left recursive grammar
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You're right - after thinking about it some more, left recursion isn't
necessary. I can define a rule for several calls in a row and then replace
it with left-associative Call tokens in the AST. IOW, the grammar wouldn't
specify the associativity at all.

Here's the updated code, and it doesn't trigger the bug, either:

CLine = ContinuedBLineFactory(Token(r'\\'))

expr0 = Token("[A-Za-z_][A-Za-z0-9_]*")

expr1 = Delayed()

def calls_to_call(tokens):
    if len(tokens) == 1:
        return tokens[0]
    elif len(tokens) == 2:
        return Call(tokens)
    else:
        return Call((Call((tokens[0], tokens[1])),
calls_to_call(tokens[2:])))

calls = expr0[2:] > calls_to_call
expr1 += calls | expr0

program = (CLine(expr1) & Eos())
program.config.default_line_aware(block_policy=rightmost)

parsed = program.parse("a b c d")
print(parsed[0])

Thanks again - I've tried using several python parsing libraries and lepl is
by far the most powerful!

Daniel Ermak

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andrew cooke  
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 More options Apr 18 2010, 12:33 pm
From: andrew cooke <and...@acooke.org>
Date: Sun, 18 Apr 2010 12:33:21 -0400
Local: Sun, Apr 18 2010 12:33 pm
Subject: Re: [LEPL] Re: "No hash" error in offside-rule left recursive grammar
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Great!  I'm glad you've got a work-around.  I'm trying to fix the bug at the
moment, but it's taking me a while to remember what it all does....

Thanks,
Andrew

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andrew cooke  
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 More options Apr 18 2010, 1:21 pm
From: andrew cooke <and...@acooke.org>
Date: Sun, 18 Apr 2010 13:21:40 -0400
Local: Sun, Apr 18 2010 1:21 pm
Subject: Re: [LEPL] Re: "No hash" error in offside-rule left recursive grammar
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OK, this is the necessary diff to fix this particular bug.  I'll release a
4.0.1 later today, probably.

diff -r 7bcfdf055100 src/lepl/stream/filters.py
--- a/src/lepl/stream/filters.py        Fri Apr 16 20:37:22 2010 -0400
+++ b/src/lepl/stream/filters.py        Sun Apr 18 13:18:24 2010 -0400
@@ -215,6 +215,12 @@
         else:
             return self.__lookup[stream.character_offset]

+    def hash_line(self, line):
+        '''
+        Extract line number from original data.
+        '''
+        return self.location(0, line, line.location_state)[0]
+

 class CachingFilteredSource(CachingTransformedSource):
     '''
[diff ends here]

You can also get the correct code from
http://code.google.com/p/lepl/source/list

Unfortunately, I still have no idea what was causing the other issue reported,
with literal strigns not being coerced to matchers using Python 2.6 on a Mac.

Andrew

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