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Comment by ossopite | Hacker News Reader

I see that you've found an example of how recursive descent parsing actually can be implemented with the visitor pattern, which I've never come across before, and I didn't read it carefully enough to understand the motivation - but that doesn't mean they are the same thing - the recursive descent parsers I've seen before just inspect which tokens are seen and directly construct AST nodes

as an adendum, the reason I don't understand the motivation is that the visitor pattern in the way I described it is useful when you have many different operations to perform on your AST. If you have only one operation on tokens - parsing into an AST - I'm not sure why you need dynamic dispatch on a second thing, the first thing being the token type. Maybe the construction is that different operations correspond to different 'grammar rules'?

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> why you need dynamic dispatch on a second thing

You're overindexing on maximally generic visitor pattern. If you have one type of visitor but nonetheless dispatch based on type that's still visitor pattern.

EDIT: to be honest who even cares. My initial point was why in the hell would you stop reading a book because a particular "pattern" offends you. And I'll reassert it here: who cares whether a recursive descent parser fits the exact definition of visitor pattern or not - you have members of a class that do stuff (construct AST nodes) and possibly track other data and then call other members. I usually call that a visitor class even if it's the only one that ever exists <shrug>

Ok, that's true, but my claim is that recursive descent parsing does not have to use the visitor pattern and indeed using recursive descent parsing is not the same as using the visitor pattern (you can do the former without the latter and I claim that you usually do)

> just inspect which tokens are seen and directly construct AST nodes

I'll repeat myself: this is not possible because you need to recursively construct the nodes (how else would you get a tree...).

I think I'm missing something here. if you have a grammar rule R with children A and B, and a function in your recursive descent parser that corresponds to R, why can R not call the parser functions for A and B, which return AST nodes themselves, and then construct another AST node using the result of those? Where was the visitor pattern required here?

Me too. No-one's denying that recursion is happening. We're just not sure about it being synonymous with the Visitor Pattern.

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