Re: [Boost-users] How to implement a visitor pattern over an AST

Subject: Re: [Boost-users] How to implement a visitor pattern over an AST
From: Michael Powell (mwpowellhtx_at_[hidden])
Date: 2018-12-11 00:27:48

On Mon, Dec 10, 2018 at 7:19 PM Larry Evans via Boost-users
<boost-users_at_[hidden]> wrote:
>
> On 12/10/18 2:14 PM, Michael Powell via Boost-users wrote:
> > On Mon, Dec 10, 2018 at 3:00 PM Steven Watanabe via Boost-users
> > <boost-users_at_[hidden]> wrote:
> >>
> >> AMDG
> >>
> >> On 12/10/2018 12:04 PM, Michael Powell via Boost-users wrote:
> >>>
> >>> I have an AST successfully building, generating text file, and
> >>> subsequently Spirit Qi parsing for verification purposes.
> >>>
> >>> Now I want to implement a visitor pattern over that AST, somehow...
> >>> For "calculators" and such, "evaluating" those expressions is one
> >>> thing; however, this AST is not that. It is closer akin to a parsed
> >>> Xml or Json model/tree.
> >>>
> >>> In other words, I'd like for it to feel sort of like an iterator if
> >>> that's even possible. It should have contextual awareness where it is
> >>> at all times, etc, perhaps "incrementing" is a depth first analysis.
> >>>
> >>
> >> It's possible to do this, but making an iterator
> >> over a tree is quite a bit more complex than just
> >> using a depth-first visitation whenever you need
> >> to process the tree.
> >
> > It is; I actually have a similar pattern in a "string rendering
> > visitor" in which I have internally specialized template methods for
> > the particular nodes of visitation. One might call that a unary
> > visitor, given a single operand.
> >
> > In this case, I rinse and repeat given the binary case, two operands:
> >
> > template<typename T>
> > bool equals(const T& x, const T& y) const {
> >      const auto message = "Comparator type '" +
> > std::string(typeid(T).name()) + "' unsupported";
> >      throw std::exception(message.c_str());
> > }
> >
> > template<>
> > bool equals<ast::TopLevelType>(const ast::TopLevelType& x, const
> > ast::TopLevelType& y) const {
> >      // ...
> > }
> >
> > And so on... Which also guarantees that I am also walking both tree's
> > at the same moment and in like fashion.
> >
> [snip]
> Hi Michael,
>
> I'm spitballing here, but my novice view of karma is that it takes an
> AST and, like your "string rendering visitor" turns it into a string
> on std::cout or some other ostream.  So, I'm guessing you could
> look at karma to see how it does it, and duplicate that method,
> only instead of rendering to a ostream, you'd just "render" as a bool
> result.
>
> Again, I could largely misunderstand what karma does, so take the
> above with a "large grain of salt".

No worries, yes, I did consider Karma for this purpose, but I figured
it may be "simple enough" to approach it otherwise. I may change my
mind downstream, however. ;-)

> -regards,
> Larry
>
>
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