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# Chapter 9: History of Leo

This chapter discusses the history of Leo and its various incarnations. A large part of this chapter discusses the process by which I discovered how to implement @file trees.

Beginnings
Versions, and more versions
Designing @file trees

  Deciding to do Leo2
  A prototype
  User interaction
  The write code
  The read code
  The load/save code
  Attributes, mirroring and dummy nodes
  Clones
  Error recovery, at last

## Beginnings

Leo grew out of my efforts to use Donald Knuth's "CWEB system of Structured documentation." I had known of literate programming since the mid 1980's, but I never understood how to make it work for me: I was confused about when to create new sections and it was difficult to keep track of all the sections once they were created.

In November 1995 I started thinking about literate programming in earnest. Over the holidays I mused about making literate programs more understandable. In January 1996 the fog of confusion suddenly cleared. I summarized my thinking with the phrase, "Webs are outlines in disguise". I strongly suspected that outline views were the key to literate programming, but many details remained obscure. It still wasn't clear exactly how to represent webs in an outline.

March 5, 1996, is the most important date in Leo's history. While returning from a day of skiing, I discussed my thoughts with Rebecca. During that conversation I realized that I could use the `MORE` outliner as a prototype for a "literate outliner." I immediately started work on my first literate outline. It quickly became apparent that outlines work: all my old problems with literate programming vanished. Furthermore, I realized that `MORE`'s outlines could form the basis for Leo's screen design. Rather than opening body text within the outline, as `MORE` does, I decided to use a separate body pane.

In about a week I hacked a translator called `M2C`which allowed me to use `MORE` to write real code. I would write code in `MORE`, copy the text to the clipboard in `MORE` format, then run `M2C`, which would tangle the outline into C code. This process was useful, if clumsy. I called the language used in the outline SWEB, for simplified CWEB. Much later I converted to the noweb language. noweb is simpler than SWEB, more standard and more general.

## Versions, and more versions

Throughout 1996 I created a version of Leo on the Macintosh in plain C and the native Mac Toolbox. This was a poor choice; I wasted a huge amount of time programming with these primitive tools. However, this effort convinced me that Leo was a great way to program.

Late in 1997 I wrote a Print command to typeset an outline. Printing (Weaving) is supposedly a key feature of literate programming. Imagine my surprise when I realized that such a "beautiful" program listing was almost unintelligible; all the structure inherent in the outline was lost! I saw clearly that typesetting, no matter how well done, is no substitute for explicit structure.

Starting in May of 1998 I began work on a version of Leo using objective C and Apple's Yellow Box environment. In some ways, this was the most powerful version of Leo. Alas, a year later Apple chose not to support Yellow Box on Windows, so I had to begin again. I was most unhappy.

In May of 1999 I began work on the Borland version of Leo for Windows. This version was successful, and I continue to support the Windows/Borland version to the present day. The Borland Delphi classes are a pleasure to use and free of bugs. I redesigned Leo's file format for the Windows version of Leo; the Yellow Box file format is a binary format that requires the Yellow Box runtime. Fortunately, I choose to use XML for Leo's file format. I have Marc-Antoine Parent to thank for this decision; he urged me to use XML and patiently explained how to use XML properly. However, there are two significant problems with the Borland version of Leo. First, it works only on Windows. Second, it can never be Open software, because it uses Borland's Delphi classes and a commercial syntax coloring component.

In May of 2000 I began work an wxWindows version of Leo. This was supposed to be a cross-platform version. Alas, after much work I chose to abandon the wxWindows version. There were too many bugs and too many incompatibilities between platforms. Something good did come from this effort. I spent a lot of time adding Python scripting to the wxWindows code and I became familiar with Python and its internals.

In October of 2001 I began work on the leo.py, an Open Software version of leo.py, a version of Leo written in Python and Tk. At last I have found the proper platform for Leo. leo.py naturally supports scripting in Python. The combination of Python and Tk is incredibly powerful, very easy to use, and truly cross platform. I rewrote Leo in Python in about two months! For the first time in my career I no longer am anxious while programming; it simply isn't possible to create bad bugs in Python.

## Designing @file trees

The following sections give a pseudo-chronological list of the major Aha's involved in creating Leo2. These Aha's form the real design and theory of operation of Leo. See the "Diary", "Notes" and "Letters to Speed Ream" sections in LeoDocs.leo for a more accurate and less tidy history of Leo2.

I am writing these notes for several reasons. First, the initial design and coding of Leo2, spanning a period of about 8 weeks, was some of the most creative and rewarding work I have ever done. The result is elegant and simple. I'm proud of it. Second, much of the design work is not reflected in the code, because improved design often eliminated code entirely. The final code is so elegant that it obscures the hard work that created it. Third, you must understand this design in order to understand the implementation of @file trees and their derived files. Someday someone else may take charge of Leo. That person should know what really makes Leo2 work.

### Deciding to do Leo2

In the summer of 2001 I began work on a project that for a long time I had considered impossible. I had long considered that "private" file formats such as .leo files were the only way to represent an outline properly and safely. I'm not sure exactly what changed my mind, but I finally was willing to consider that information embedded in derived files might be useful. This meant accepting the possibility that sentinel lines might be corrupted. This was a crucial first step. If we can trust the user not to corrupt sentinel lines than we can embed almost any kind of information into a derived file.

There were several motivations for this work. I wanted to eliminate the need for explicit Tangle and Untangle commands. I thought of this as "Untangle on Read/Tangle on Write." If tangling and untangling could be made automatic it would save the user a lot of work. I also wanted to make derived files the primary sources files. .leo files might be made much smaller derived files contained the primary source information. This hope turned out to be false.

The result of this design work was something I originally called Leo2, though I now usually prefer to talk about @file trees. Initially most design issues were unresolved or unknown. I resolved to attempt a robust error-recovery scheme, not knowing in advance what that might involve. I also wanted to solve what I thought of as the "cross-file clone" problem: clones that point from a .leo outline into a derived file. With Leo1 cross-file clones do not exist; everything is in the same .leo file. It was clear that Leo2 would have to change some aspects of clones, but all details were fuzzy.

### A prototype

The next step was also crucial. I started to use Leo1 as a prototype to design what the new body pane would look like to the user. In retrospect, using Leo1 as a prototype for Leo2 was just as inspired as using `MORE` as a prototype for Leo1. Both prototypes marked the true beginning of their respective projects. The Leo2 prototype was a mockup in Python of the code for reading and writing derived files. The file LeoDocs.leo contain these first prototype nodes.

Writing the prototype got me thinking about improving noweb. With my experience with Leo1, I was able to create a new markup language that took advantage of outline structure. I called the new language "simplified noweb", though that terminology is obsolete. I created @file nodes to distinguish between the old and new ways of creating derived files. In Leo1, the @code directive is simply an abbreviation for a section definition line. Simplified noweb used @c as an abbreviation for @code. More importantly, simplified noweb used @c to separate doc parts from code parts without necessarily specifying a section name. It quickly became apparent that most nodes could be unnamed. All I needed was the @others directive to specify the location for all such unnamed nodes.

From the start, simplified noweb was a joy to use. Indeed, the @others directive could replace all section definition lines. Furthermore, I could make @doc directive optional if the body pane started in "code mode". But this meant that plain body text could become a "literate" program! This was an amazing discovery. These Aha's got me excited about Leo2. This was important, as it motivated me to do a lot of difficult design work.

### User interaction

In spite of this excitement, I was uneasy. After much "daydreaming" I realized that I was afraid that reading and writing derived files would be interrupted by a long series of alerts. I saw that designing the "user interaction" during reading and writing would be very important. The next Aha was that I could replace a long series of alerts with messages to the log window, followed by a single "summary" alert. Much later I saw how to eliminate alerts entirely.

At this time I thought there would be two kinds of "errors" while reading derived files. Warnings would alert the user that something non-serious had happened. True errors would alert the user that data might have been lost. Indeed, if Leo2 saves orphan and ignored nodes in a .leo file under an @file node, then read errors could endanger such nodes. Much later I saw that a robust error recovery scheme demands that @file nodes not contain orphan and @ignored nodes. (More on this subject later.) But if orphan and @ignored nodes are moved out of @file trees, there are no read errors that can cause data loss! So the distinction between warnings and errors finally went away.

### The write code

I next turned my attention to writing @file nodes. A huge Aha: I realized that sentinel lines must contain both a leading and a trailing newline. The general principle is this: the write code must contain absolutely no "conditional" logic, because otherwise the read code could not figure out whether the condition should be true or false. So derived files contain blank lines between sentinel lines. These "extra" newlines are very useful, because the read (untangle) code can now easily determine exactly where every blank, tab and newline of the derived file came from. It would be hard to overstate how important this simplifying principle was in practice.

Much later, with urging from a customer, I realized that the write code could safely remove "extra" newlines between sentinels with a caching scheme in the low level atFile::os() routine. This scheme does not alter the body of the write code in any way: in effect, sentinels still contain leading and trailing "logical" newlines. The read code had to be modified to handle "missing" leading newlines, but this can always be done assuming that sentinels still contain logical leading and trailing newlines!

At about this time I designed a clever way of having the write code tell the read code which newlines were inserted in doc parts. (The whole point of doc parts is to have the write code format long comments by splitting long lines.) To quote from my diary:

> "We can use the following convention to determine where putDocPart has inserted line breaks: A line in a doc part is followed by an inserted newline if and only if the newline is preceded by whitespace. This is a really elegant convention, and is essentially invisible to the user.

> Tangle outputs words until the line would become too long, and then it inserts a newline. To preserve all whitespace, tangle always includes the whitespace that terminates a word on the same line as the word itself. Therefore, split lines always end in whitespace. To make this convention work, tangle only has to delete the trailing whitespace of all lines that are followed by a 'real' newline."

### The read code

After the write code was working I turned my attention to the read (untangle) code. Leo's Untangle command is the most complex and difficult code I have ever written. Imagine my surprise when I realized that the Leo2 read code is essentially trivial! Indeed, the Leo2 untangle code is like an assembler. The read code scans lines of a derived files looking for "opcodes", that is, sentinel lines, and executes some simple code for each separate opcode. The heart of this code is the scanText routine in atFile.cpp.

The read code was written and debugged in less than two days! It is the most elegant code I have ever written. While perfecting the read code I realized that sentinel lines should show the complete nesting structure found in the outline, even if this information seems redundant. For example, I was tempted to use a single sentinel to represent an @other directive, but finally abandoned this plan in favor of the @+other and @-other sentinels.

This redundancy greatly simplified the read code and made the structure of derived files absolutely clear. Moreover, it turned out that we need, in general, all the information created by the present sentinel lines. In short, sentinels are as simple as they can be, and no simpler.

The atFile::createNthChild method is a very important: it ensures that nodes will be correctly inserted into the outline. createNthChild must be bullet-proof if the Read code is to be robust. Note that the write code outputs @node sentinels, that is, section definitions, in the order in which sections are referenced in the outline, not the order in which sections appear in the outline. So createNthChild must insert the n'th node of parent p properly even if p contains fewer than n-1 children! The write code ensures that section references are properly nested: @node sentinels are enclosed in @node sentinels for all their ancestors in the @file tree. createNthChild creates dummy siblings as needed, then replaces the dummy siblings later when their actual definitions, that is, @node sentinels, are encountered.

At this point the fundamental read/write code was complete. I found three minor bugs in the code over the next week or so, but it was clear that the read/write code formed a rock-solid base from which to continue design and implementation. This was an entirely unexpected surprise.

### The load/save code

At this point I could read and write derived files "by hand", using temporary Read and Write commands. The next step was to integrate the reading and writing of derived files with the loading and saving of .leo files. From time to time I made minor changes to the drivers for the read/write code to accommodate the Load and Save code, but at no time did I significantly alter the read or write code itself.

The user interaction of the Load and Save commands drove the design and implementation of the load/store code. The most important questions were: "what do we tell the user?", and "what does the user do with the information?" It turns out that the user can't make any complex decision during error recovery because the user doesn't have nearly enough information to make an informed choice. In turn, this means that certain kinds of error recovery schemes are out of the question...

### Attributes, mirroring and dummy nodes

I now turned my attention to "attributes" of nodes. Most attributes, like user marks, are non-essential. However, clone information is essential; we must never lose clone links. At this time I had a preliminary design for cross-file clones that involved a two part "pointer" consisting of a full path name and an immutable clone index within the derived file. Eventually such pointers completely disappeared, but the immutable clone indices remain.

My first thought was that it would be good to store all attributes in @node sentinels in the derived file, but experience showed that would be irritating. Indeed, one wants Leo2 to rewrite derived files only if something essential has changed. For example, one doesn't want to rewrite the derived file just because a different node as been selected.

At this point I had another Aha: we can use the .leo file to store all non-essential attributes. For example, this means that the .leo file, not the derived files, will change if we select a new node. In effect, the .leo file mirrors the derived file. The only reason to store nodes in the .leo file under an @file node is to carry these attributes, so Leo2 wrote dummy nodes that do not reference body text. Much later I saw that dummy nodes were dangerous and that .leo files should contain all information found in derived files.

### Clones

The concept of mirroring created a huge breakthrough with cross-file clones: Here is an excerpt of an email i sent to my brother Speed:

> "I realized this morning that since a .leo file contains dummy vnodes for all nodes in a derived file, those dummy nodes can carry clone info! I changed one line to make sure that the write code always writes clone info in dummy vnodes and voila! Cross-file clones worked!"

All of Leo1's clone code could be used completely unchanged. Everything "just works".

### Error recovery, at last

At first I thought we could make sure that the .leo file always correctly mirrors all derived file, but disastrous experience showed that is a completely false hope. Indeed, backup .leo files will almost never mirror derived file correctly. So it became urgent to find a completely fool-proof error recovery scheme.

I had known for quite a while that error recovery should work "as if" the mirroring nodes were deleted, then recreated afresh. Several failed attempts at an error recovery scheme convinced me that error recovery would actually have to delete all dummy nodes and then do a complete reread. This is what Leo2 does.

But erasing dummy nodes would destroy any orphan and ignored nodes--by definition such nodes appear nowhere in the derived file. Therefore, I had to enforce the rule that @file nodes should contain no such nodes. Here is an email I wrote to my brother, Speed Ream discussing what turned out to be the penultimate error recovery scheme:

> "The error recovery saga continues. After much pondering and some trial coding I have changed my mind about orphans and @ignored nodes. They simply should never appear as descendants of @file nodes. Fortunately, this simplifies all aspects of Leo2.

> Leo2 will issue a warning (not an error) if an orphan or @ignored node appears as the descendant of an @file node when a .leo file is being saved. If any warnings occur while writing the derived file, Leo2 will write the "offending" @file tree to the .leo file instead of the derived file. This has several advantages:

> 1. The user gets warned about orphan nodes. These are useful warnings! Orphan nodes arise from missing @others directives or missing section references.

> 2. The user doesn't have to change anything immediately in order to save an outline. This is very important. Besides warnings about orphans, Leo2 will also warn about undefined or unreferenced sections. User's shouldn't have to fix these warnings to do a Save!

> 3. No errors or alerts will occur during Reading or Writing, so the user's anxiety level goes way down. At worst, some informational message will be sent to the log. The user will never have to make important decisions during Loads or Saves. [At last the dubious distinction between errors and warnings disappears.]

> 4. Error recovery can be bullet-proof. Simple code will guarantee that after any read operation the structure of an @file node will match the structure of the derived file. Also, sentinels in derived files will now account for all children of an @file node. There are no more "missing nodes" that must be filled in using the .leo file. Finally, error recovery will never change the @file tree in any way: no more "recovered nodes" nodes.

> 5. The present read code can be used almost unchanged. The only addition is the posting of a warning if the structure of the .leo file does not match the structure of the derived file. We need a warning because non-essential attribute of nodes (like user marks) may be altered."

This ends the original history of Leo2. In fact, it took quite a while before Leo recovered properly from all errors. I finally saw that .leo files should duplicate all information in derived files. This allows a .leo file to be used a single backup file and allows maximal error recovery in all situations. It took several months to stamp out several subtle bugs involving clones that caused spurious read errors. Such errors undermine confidence in Leo and can cause disastrous reversions. See my diary entries for January 2002 in leo.py for details.