[mercury-users] Bug in lexer.m

Julien Fischer juliensf at csse.unimelb.edu.au
Wed Jul 16 18:45:32 AEST 2008

On Tue, 15 Jul 2008, Nicholas Nethercote wrote:

> On Tue, 15 Jul 2008, Nicholas Nethercote wrote:
>> :- pred string_ungetchar(string::in, posn::in, posn::out) is det.
>> string_ungetchar(String, Posn0, Posn) :-
>>    Posn0 = posn(LineNum0, LineOffset0, Offset0),
>>    Offset = Offset0 - 1,
>>    string.unsafe_index(String, Offset, Char),
>>    ( Char = '\n' ->
>>        LineNum = LineNum0 - 1,
>>        Posn = posn(LineNum, Offset, Offset)
>>    ;
>>        Posn = posn(LineNum0, LineOffset0, Offset)
>>    ).
>> In a 'posn', the first arg is the current line number, the 3rd arg is the 
>> current offset into the string being parsed, and the 2nd arg is the offset 
>> of the start of the current line.
>> When the above code goes back over a newline, the 2nd argument is 
>> incorrectly set -- it gets set to 'Offset', which is the offset of the 
>> newline character that ended the line.  It should be set to the offset of 
>> the beginning of that line (which will be a number of characters earlier 
>> than 'Offset').  But by this point the offset for the start of that line 
>> has been lost and cannot be easily recreated.
>> I suspect this has been wrong for a very long time, but nobody has noticed
>> because the lexer doesn't ever use the 2nd field anywhere (it could be used 
>> to report column numbers for lexing errors, but it isn't).  So perhaps the 
>> 3-argument 'posn' type could be replaced with a 2-argument type, and some 
>> memory would be saved.
> And some more savings could be made if that 2-argument type was split into 
> two parts which were passed around separately.  It results in extra arguments 
> to lots of lexing predicates, but avoids the deconstruct/construct pair which 
> currently occurs on every character.
> I just made a change like this to the Zinc lexer, which is structured 
> similarly, and got a nice speedup (I think it was about 20--30%, although I 
> didn't measure it all that carefully).

Thanks for reporting this!

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