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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: Wed, 19 Dec 2018 15:38:33 +0100

Hans Åberg wrote:

> > By default, I keep recipients (list) and CCs (Akim) as is.
> With a reply to all, it would come out right.

Depends on what you consider right. It's a mailing list. Those who
write here are expected to read via the list (subscription or
archive; I know you do) or take their own measures (Reply-To or CC).
With "reply to all", I get all of your mails twice, doesn't seem
right to me (though only slightly annoying these days, as opposed to
20 years ago when every KB of internet data cost a fortune ;).

> > If you
> > want a private copy, set a Reply-To.
> Those are generally bad.

I don't agree. Some years ago there was a somewhat famous rant
against them, but it was full of BS and misunderstandings.

Or you can set a CC to yourself if you wish.

> I might not notice your posts, so do as you wish.

Honestly, you *not* noticing my posts is one of my smallest worries. ;)

> >>>> 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.
> It was in response to your comment.

Which was in response to your comment about some mysterious "extra

> There might be a problem with the Bison variant if one in the
> lexer returns an object of another type that the token type, and
> expect a conversion.

There is no conversion, and in most cases, there couldn't be any
when types are too different; but even if types are similar enough
that a safe automatic conversion would be possible, say char to int,
I know of no variant or union that would do so. That's not specific
to Bison's variant.

So such a mismatch is an error. The only difference is how this
error is handled. std::variant throws an exception on usage; Bison's
variant so far basically makes it UB (which is a valid answer; many
things in C++ are UB), the current proposal would report an error on
construction already, so slightly better than std::variant (in case
the mismatching value is silently discarded).


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