Re: Lexical feedback and lookahead

[Hans Aberg]

On 19 Jul 2005, at 17:01, Evan Lavelle wrote:

> The problem is, in my input, there aren't *any* context-independent  
> keywords (not in this case, anyway). This is equivalent to the  
> COBOL problem where FUNCTION is optional. The grammar looks like this:
>
> program : function_list ;
>
> function_list
>  : /* nothing */
>  | function_list function ;
>
> function : { recognise_function_names = true; }
>   valid_function '{' function_body '}' ;
>
> valid_function : FOO | BAR ;
>
> The only way that I know a new function is coming up is that an  
> existing function has just completed: there's no convenient keyword  
> to give me warning. [If my grammar really was this simple, then I  
> could probably use another token to recognise the end of a  
> function, but the real input is pretty complex].

It seems me that you know that a function has been completed when the  
function body top level "{" ... "}" has completed. This can be kept  
track of by a bracket depth count in the lexer.

> Any ideas? It seems to me that I need an 'unget' implementation.

In Bison, it is called %glr.

  Hans Aberg