[antlr-interest] a simple (not for me :)) grammar problem

[Fırat Küçük]

NoViableAltException :(

i guess we cannot use syntactic predication over LEXER rules.


2008/1/7, Mark Volkmann <r.mark.volkmann at gmail.com>:
> On Jan 6, 2008 5:31 PM, Fırat Küçük <firatkucuk at gmail.com> wrote:
> > yes i need it.
>
> I decided to take a shot at figuring out a solution to your grammar.
> It should be easy right. Terr already gave the hint that the problem
> is that it was greedily grabbing the DOT for FLOAT instead of leaving
> it for the separator between the number and the identifier. Piece of
> cake? Well I've tried several things I thought would work to no avail.
> Why in the world doesn't this work?
>
> grammar Sample;
>
> options {
>  backtrack = true; // I shouldn't need this, but I don't think it can hurt.
> }
>
> start
>  // Using a syntactic predicate to order alternatives.
>  : (INTEGER DOT IDENTIFIER)=> INTEGER DOT IDENTIFIER
>  | FLOAT DOT IDENTIFIER  ;
>
> FLOAT: NUMBER DOT NUMBER;
> INTEGER: NUMBER;
> IDENTIFIER: LETTER+;
> DOT: '.';
> fragment NUMBER: DIGIT+;
> fragment LETTER: 'a' .. 'z';
> fragment DIGIT: '0' .. '9';
>
> > 07.01.2008 tarihinde Mark Volkmann <r.mark.volkmann at gmail.com> yazmış:
> > > On Jan 6, 2008 5:23 PM, Fırat Küçük <firatkucuk at gmail.com> wrote:
> > > > hi,
> > > > the problem is related to following grammar:
> > > >
> > > > grammar Sample;
> > > >
> > > > start: (FLOAT | INTEGER) DOT IDENTIFIER;
> > > >
> > > > FLOAT: NUMBER DOT NUMBER;
> > > > INTEGER: NUMBER;
> > > > IDENTIFIER: LETTER+;
> > > > DOT: '.';
> > > > fragment NUMBER: DIGIT+;
> > > > fragment LETTER: 'a' .. 'z';
> > > > fragment DIGIT: '0' .. '9';
> > > >
> > > > let us try:
> > > >
> > > > 3.hello
> > > >
> > > > result:
> > > > line 1:2 required (...)+ loop did not match anything at character 'h'
> > > > line 1:3 mismatched input 'ello' expecting set null
> > >
> > > I think it's trying to match a FLOAT which requires a number after the
> > > DOT. It should work if you change the start rule to this.
> > >
> > > start: INTEGER DOT IDENTIFIER;
> > >
> > > Do you really need to be able to parse something like this?
> > > 3.14.hello
>
> --
> R. Mark Volkmann
> Object Computing, Inc.
>


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ADAMYO Distance Learning
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