Re: Get allowed-next-tokens in yacc?

"R Faulds" <rfaulds@avici.com> writes:


> 1) Is there a way (without hacking y.tab.c) to get the list of allowed
> tokens for some (any) point in the parse process?


Sure. This would be yacc-dependent though.


> >show interface<help-key>
> <num> - numeric input required
> ALL - display all


This sounds good, but for some tokens (the "syntactic noise" tokens)
you can't give sensible help. Eg '(' may mean signify the beginning of
an "argument list" or a "nested expression", and ALL may mean "all
interfaces or "all widgets", etc.


> 2) When I'm parsing down a specific rule, how do I use error to localize
> the error handling?


> >set interface address <CR>
> trap the error at the rule's action code so I could issue
> "numeric required, choose an interface between [1..7]".


This is related to the previous point. If you can figure out the set
of allowed tokens, you can make a sensible error message.


> [Honestly, I wouldn't use yacc. Getting the actual list of allowed tokens
> at each point is really hard due to the state and table compaction that
> yacc does. And the error recovery is no prize-winner. -John]


I don't know what compaction yacc does, but I'd suggest you use bison.
Look at the skeleton file bison.simple, after a "#ifdef
YYERROR_VERBOSE" label. If that is defined, bison calls yyerror with
eg "parse error, expecting `foo' or `bar' or `'+'' or `NUM' or
'ALL'". It's a simple matter to enrich this with more detailed
explanations taken from the table containing the help texts. Note that
some of the expected tokens could be non-terminals.
--

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