Code Outlines

Pavel Panchekha

You may have noticed that Web Browser Engineering’s chapters now have outlines listing all of the classes, constants, and methods described in the book. That’s a feature I’ve been working on for a while, and I wanted to share how it works.

Why outlines

One of the challenges both reading and writing Web Browser Engineering is tracking how the code changes over time. The eleven chapters of Parts 1–3 write about 2000 lines of code, which total to about 1000 when you net out edits and deletions. That’s small by browser standards (hah!) but still a sizable project, where how you organize it starts to matter.

Besides, the point of Web Browser Engineering isn’t just to build a working browser from scratch. It’s also to explain how browsers are structured internally and what they spend all their time doing—the sort of knowledge that could make you a better web developer even if you never plan to touch a browser code base. Again, for this purpose, a high-level overview of how a browser is structured helps a lot.

I saw both of these factors when teaching last year. Over the course of a few months, students see many different parts of a browser. When it comes time to edit and refactor (like in Chapter 10) it’s easy to have only a hazy recollection of how the code you’re refactoring actually works. And since I wanted my students to come away with an understanding of a browser’s high-level components, I wanted to give them a quick way to view the completed browser at an intermediate level of detail somewhere between the full code listing and the book’s table of contents.

Outlining code

Code outlines are a way to get that intermediate level of detail directly from the code. It’s inspired by code-folding as implemented in many text editors, reducing functions and classes to single lines. Plus, in this book’s coding style, individual functions directly refer to specific browser concepts and are usually organized into classes that refer to specific browser components.

As far as I know, code folding (like a lot of text editor tools) is generally based on regular expressions. So my first attempt at code outlines was literally to grep the code for lines beginning with def or class. That worked fairly well to identify functions and classes, but gave the function definition lines unchanged, like this one from the current Chapter 11 draft:

def load(self, url, body=None):

I don’t really want to show all of this. Default values aren’t important when you’re trying to get a birds-eye view of the code, and the self parameter that Python requires might be pedagogically useful when writing code, but it’s redundant when you’re looking at an outline where methods and functions are clearly distinct.

I could fix some of these by further mangling the text with regular expressions, but then I remembered that Python comes with an ast library that exposes the Python parser itself as a Python library. Using a parser means I don’t need ugly regular expressions, and guarantees that I don’t have to keep the limitations of those regular expressions in mind as I edit the text.

Using the ast library is also really easy: I call ast.parse and then walk the tree looking for ClassDef, FunctionDef, and Assign expressions, plus those unusual if statements that are Python’s version of int main(). For classes I walk the class body looking for function definitions, and for function definitions I extract the argument names but not their values with:

args = [arg.arg for arg in stmt.args.args if arg.arg != "self"]

I put all this logic into a little program, which takes the Python file it outlines as an argument. Because I’ve got a copy of the browser code at the end of each chapter (as files named lab1.py, lab2.py, and so on) I can use the outliner to get an outline for each chapter.

Displaying the outlines

I want code outlines at the end of every chapter. But the book itself is written in Markdown and converted to static HTML with Pandoc, so I need to somehow run my parser/outliner inside Pandoc. Luckily, Pandoc is scriptable in LuaFirst time I’ve had to use the language, so this part was pretty slow-going. and also supports custom blocks, which can furthermore be adorned with classes. That means I can write a Pandoc plugin that detects blocks with the cmd class and acts on them specifically:

function Div(el)
  if el.classes[1] == "cmd" then
    -- ...
  else
    return el
  end
end

Pandoc will run this Div function on every custom block in the file, and replace the actual block el with whatever this function returns. I’ll be replacing these cmd blocks with the results of running a command like my parser/outliner.

In Pandoc, Div objects contain other block-level objects like paragraphs and code blocks. To avoid problems with punctuation and inline markup, I decided that the actual command should be inside a code block:

if #el.content ~= 1 or
   el.content[1].t ~= "CodeBlock" then
  error("`cmd` block does not contain a code block")
end
local cmd = el.content[1].text

Here #el.content gets the length of the content list and ~= is Lua’s not equals operator. Once the command is extracted, the popen function can be used to run the command:

local proc = io.popen(cmd)
local results = proc:read("*all")

I’m not a Lua expert, but I think the colon means calling a function as a method, while the dot means calling it like a namespaced member. The *all argument to read is a special argument that tells read to read the whole contents of the file descriptor.

Finally, I want to insert the results back into the Pandoc document. That means returning a new Pandoc object from Div with the outliner results. Initially, I returned a Pandoc CodeBlock object, which is its name for source code:

local pre = pandoc.CodeBlock(results)
pre.classes = el.classes
return pre

By copying classes from the original custom block to the new code block, the new code block would have a cmd class that I could use to add two columns, and I could also add a python class to add syntax highlighting and other classes like outline to specify that this specifically is an outline command.

Styling the results

The outlines are fairly long. The current final copy of the code has about 100 functions in 18 classes, and as I add more content that might grow. But since I’ve only got argument names, each line is pretty short; the longest one right now is 46 characters and it’s an outlier. So to make things more digestible I want to show the outline in two columns, which is pretty easy in CSS:

.outline { font-size: 60%; font-family: monospace; column-count: 2; }

The small font size works out to 15 pixels, small but still readable. And remember, all the information in the outline is still available in the other code blocks.

But two columns had a problem: the column break could happen in the middle of a class, which would look ugly and also lead to confusion about which function definitions are indented (because they are methods) and which are not. The break-inside property in CSS prevents that, but I didn’t have any HTML element around individual classes, since I was presenting the output as a source block.

The only way I could think of to fix this (other than rearranging my code so that column breaks only occur between classes) was to produce richer output than raw text. In Pandoc you can use the RawBlock construct to do that:

pre = pandoc.Div({ pandoc.RawBlock("html", results) })

A RawBlock is ignored unless Pandoc is converting to the named format, so if I ever want to publish this book as an EPUB or PDF file, I’ll need to continue working on this bit of the code, but for now HTML is enough. Outputting HTML meant the output wouldn’t be syntax-highlighted any more, but luckily the outliner itself already has access to a parsed AST, so I could add the syntax highlighting myself:

class IfMain:
    def html(self):
        str = "if __name__ == \"__main__\""
        return str.replace("if", "<span class=cf>if</span>") \
            .replace("==", "<span class=op>==</span>") \
            .replace("\"__main__\"", "<span class=st>\"__main__\"</span>")

Each top-level object in the outline is wrapped in a code.line element, and I add the break-inside property to that. I also add a little bit of whitespace between the top-level elements (which are mostly classes) and indent class contents by two spaces:

.outline code { break-inside: avoid; display: block; }
.outline > code { margin: .33em 0; }
.outline code > code { margin-left: 2ch; }

A benefit of producing my own HTML is that I can also stray a bit further from Python syntax. I already don’t output colons for function definitions (to save space and since they don’t have a body anyway), but I’ve been thinking of merging constructors with the class header too. Right now the final chapter’s outline is about two screens of text, and if by some tricks I can cut it down to one that would be fantastic.