Understanding the AST...

[RazvanN]

Hi Joe,

I suggest you watch this video which explains how the parse time 
visitors work: https://www.youtube.com/watch?v=tK072jcoWv4 .

On Tuesday, 6 February 2018 at 12:03:06 UTC, joe wrote:
> Hello everybody!
>
> Last week end I found this post ( 
> https://dlang.org/blog/2017/08/01/a-dub-case-study-compiling-dmd-as-a-library/ ) on the Blog and thought to myself awesome.
>
> So I built the library and everything went smooth. Thanks for 
> the effort of all the involved people who made that possible!
>
> I've had a look at the 2 examples, too, the avg. function lines 
> ( https://github.com/dlang/dmd/blob/master/src/examples/avg.d ) 
> and the import ( 
> https://github.com/dlang/dmd/blob/master/src/examples/impvisitor.d ) ones and for a start I decided to make a program that prints the outline of a module.
>
> Turns out I don't really understand how to access the data in 
> the AST.
> For everything there's a visitor method and overriding a few of 
> them to print return statements and some such works as 
> advertised.
>
> However, I have no idea where I am in the tree when any of 
> those methods are called.
> Like for example in 
> FunctionLengthVisitor(AST).visitFuncBody(AST.FuncDeclaration 
> fd).
> I have a function declaration object which tells me everything 
> about what's inside the function, but how do I know what or 
> where this function belongs to, where can I get that 
> information ? I don't see anything about UDAs either, nor the 
> doc comment.
>

The FuncDeclaration node contains all the information for that.
For example, you can access fd.parent to see if the function is
declared at top-level (in which case, the parent is going to be a 
module
declaration ) or if it is a nested function (in a class, in a 
struct, in a function).
Every AST node contains information about the position in the 
AST, all you
have to do is find how to get that information: which field to 
access or which
member function to call.

> I understand when visitor.getAvgLen is called with the parsed 
> module, the accept function calls a visitor overload for each 
> member.
> But this sounds to me like I'd have to do a lot of book keeping 
> in my visitor to keep track of things which are already present 
> in the AST.

The function average length visitor inherits a transitive visitor
which means that the AST traversal logic is already implemented 
for you.
All you have to do is override the visiting methods of interest 
and do
whatever suits you : print stuff, alter the ast, stop the 
visitation or
continue the visitation (by calling super.visit(ASTnode)).

>
> Any insight to this would be much appreciated :)

I know that my explanations might not be very explicit, but if 
you have an example please post it and we can work on it.

Cheers,
RazvanN