Re: Why is this a shift/reduce conflict?

[Adam Smalin]

The problem with making >> one token is the problem C++ templates has with
`map<int, list<int>>`. I know it can be changed to be something like D
which i think is map!(int, list!(int)) but I'd like to have the option of
doing it in C++ which i may not take.


On Tue, Jun 4, 2013 at 8:32 PM, Valentin Tolmer <address@hidden>wrote:

> On Wed 05 Jun 2013 12:06:30 AM CEST, Adam Smalin wrote:
>
>> During my test to get  << a difference precedence then < I suspect it is
>> impossible because after expr < expr [<] it has no idea if it will be a <<
>> expr or an < expression thus I can't tell it to have a different
>> precedence
>> between < and < <.
>>
>> Because I can't I wrote this test grammar. When I execute `a<b< <cd<e` I
>> get << < < as I wanted. The issue is 2 shift reduce conflicts. I can't
>> understand it. It does exactly what I want (<< before <). Its has
>> %glr-parser at the top. Why isn't it ok shifting and reducing when it
>> can't
>> complete the longer rule (mainElement3). I'm just confused all over I
>> don't
>> see the conflict. How would I rewrite this to not have the conflict?
>>
>> %nonassoc DOLLAR
>> //%left '<'
>> %left '*' PREC
>> %left '.'
>>
>>
>> %%
>> program: mEOS main
>> main: | mainLoop mEOS
>> mainLoop:
>>        mainElement
>>      | mainLoop mainElement
>> mainElement:
>>        mainElement3
>>      | mainElement '<' mainElement3    { printf("<\n"); }
>> mainElement3:
>>        mainElement4
>>      | mainElement3 '<' '<' mainElement4     { printf("<<\n"); }
>> mainElement4:
>>        '$' VarName     %prec DOLLAR    { printf("varname\n"); }
>>      | mainElement2
>>      //| mainElement '*' mainElement    { printf("*\n"); }
>> mainElement2:
>>        VAR             { printf("var %s\n", yytext); }
>>      | Token         { printf("token\n"); }
>>
>> Token: '.'
>> VarName: VAR
>>      | VarName  '.' VAR
>>
>> EOS:   '\n' | ';'
>> mEOS:       | mEOS EOS
>>
>> %%
>>
>>
>> state 12
>>
>>      6 mainElement: mainElement3 .  [$end, VAR, '.', '<', '$', '\n', ';']
>>      9 mainElement3: mainElement3 . '<' '<' mainElement4
>>
>>      '<'  shift, and go to state 22
>>
>>      '<'       [reduce using rule 6 (mainElement)]
>>      $default  reduce using rule 6 (mainElement)
>>
>> state 24
>>
>>      7 mainElement: mainElement '<' mainElement3 .  [$end, VAR, '.', '<',
>> '$', '\n', ';']
>>      9 mainElement3: mainElement3 . '<' '<' mainElement4
>>
>>      '<'  shift, and go to state 22
>>
>>      '<'       [reduce using rule 7 (mainElement)]
>>      $default  reduce using rule 7 (mainElement)
>> ______________________________**_________________
>> address@hidden 
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>>
>
> If this is a possibility, you could detect a double << at the lexer level,
> and give the parser a special token corresponding to <<. That would
> simplify your grammar.
>