Re: [Boost-users] Spirit: how to force backtracking

Subject: Re: [Boost-users] Spirit: how to force backtracking
From: Felipe Magno de Almeida (felipe.m.almeida_at_[hidden])
Date: 2017-11-03 18:32:19

On Fri, Nov 3, 2017 at 8:20 AM, Stian Zeljko Vrba via Boost-users
<boost-users_at_[hidden]> wrote:
> I have the following grammar rule:
>
>
> value = (int_ % ',') | *ascii::print;
>
> The intention is to parse either a comma-separated list of integers or a
> string. The rule as such fails in cases like "123ABC", which should be
> parsed as string.
> From the description of the | operator, I would expect the parser to
> backtrack and try the other alternative when it sees that the first
> alternative does not match the input. However, I get a parse error instead
> (this rule is a part of a larger grammar), I guess because the first
> alternative matches the single integer but the rest of the grammar cannot be
> matched.
>
> How to rewrite the grammar to resolve this ambiguity, or force spirit to
> backtrack if the first alternative of 'value' cannot be matched?

It seems that "123" is a valid comma-separated list of integers (with 1 integer)
which gets matched. Maybe you could use an eps rule to match a separator
without consuming the input.

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Regards,
-- 
Felipe Magno de Almeida