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

On Mon, Dec 10, 2018 at 7:19 PM Larry Evans via Boost-users wrote:

On 12/10/18 2:14 PM, Michael Powell via Boost-users wrote:

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On Mon, Dec 10, 2018 at 3:00 PM Steven Watanabe via Boost-users wrote:

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AMDG

On 12/10/2018 12:04 PM, Michael Powell via Boost-users wrote:

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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 equalsast::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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