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Re: Porting to typed C++ parser (was: Dynamic token kinds)

From: Frank Heckenbach
Subject: Re: Porting to typed C++ parser (was: Dynamic token kinds)
Date: Tue, 18 Dec 2018 13:20:26 +0100

Hans Åberg wrote:

> > On 17 Dec 2018, at 19:09, Frank Heckenbach <address@hidden> wrote:
> [Note: you don't cc me, only others, for some reason.]

By default, I keep recipients (list) and CCs (Akim) as is. If you
want a private copy, set a Reply-To.

> > so:
> > 
> >> Actually, I pass the semantic value through a class object to
> >> which the lexer function belongs, so the extra arguments are not
> >> needed. So I must think more about this.
> > 
> > FWIW, my lexer function is also part of a "driver" object, but I
> > don't think that's relevant here. Maybe you think of the "%param"
> > arguments, but it's not about those.
> No, I use the Flex C++ parser right now, which for some reason
> requires an empty argument, so this works without patching the
> header, that is, until they decided to bomb it in 2.6 and later.

I don't use flex anymore, and never used flex C++, so I don't know
about that. But again, that's unrelated to the current proposal.

> > In my proposal:
> > 
> >  symbol_type make_symbol (token_type type, int &&);
> > 
> > the second parameter is actually the semantic value (and the first
> > one the token kind, of course), so there are no extra arguments, no
> > driver, no lexer, not even the parser (no hidden "this" parameter,
> > since these are static functions), so I think there's nothing to
> > worry about. It's just about building a token from these two
> > parameters, as you'd expect to, by basically calling its
> > constructor, and (as per the proposal) adding checks to help avoid
> > mismatches between the two of them.
> > 
> > But if you tell us more about how you (plan to) build your tokens,
> > we could say if there are any potential problems.
> I use a polymorphic (virtual) pointer class, a string, a token
> number, or a combination thereof depending on context. For the
> first, it might be useful with typecasts that simplify the code.

A variant is (in this regard) somewhat similar to a polymorphic
pointer (except that the pointer needs memory management, as you
know), so it seems you should be able to use a variant instead.

With variants, you usually don't use type-casts, but accessors, such
as get<> with std::variant or as<> with Bison's variant, though in
Bison parsers, that usually happens automatically with $n.

So I see no reason why you shouldn't be able to use variants, but of
course, without seeing any details, I can just talk generally.


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