Re: Features you would like to see
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- Subject: Re: Features you would like to see
- From: "Rici Lake" <lua@...>
- Date: 2007年8月18日 20:00:43 +0100 (BST)
David Given wrote:
> Depends on the state machine.
>
> For example (this is a classic example by David Tribble, not my own):
>
> int parse()
> {
> Token tok;
>
> reading:
> tok = gettoken();
> if (tok == END)
> return ACCEPT;
> shifting:
> if (shift(tok))
> goto reading;
> reducing:
> if (reduce(tok))
> goto shifting;
> return ERROR;
> }
The state machine in a shift-reduce parser is not really revealed
by that code; the states themselves are hidden in the parser's stack.
In any event, the code is incorrect; the END token needs to be handled
(causing pending productions to be reduced) before returning ACCEPT.
So it could just as well be written:
int parse() {
Token tok;
do {
tok = gettoken();
while (!shift(tok)) {
if (!reduce(tok)) return ERROR;
}
} while (tok != END);
return ACCEPT;
}
However, a more interesting implementation might use tail-calls to
the actual state resulting from the shift or reduce.