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Automatically generating railroad diagrams from yacc files

Reading and understanding a language’s syntax written in the BNF-like notation used by yacc/bison takes some practice. Railroad diagrams are a much more user friendly notation, but require a lot of manual tweaking before they look as good as the following example from the json.org website:

Lexical syntax of a numeric literal.

I’m currently working on a language whose syntax is evolving and I want to create a visual representation of it that can be read by non-yacc experts; spending a day of so manually creating a decent looking railroad diagram is not an efficient use of time. What automatic visualization tools are out there that I can use?

A couple of tools that look like they might produce useful results are web based (e.g., bottlecaps.de; working on an internal project for a company means I cannot take this approach). Some tools take EBNF as input (e.g., my28msec.com which is also online based); the Extensions in EBNF obviate the need for many of the low level organizational details that appear in grammars written with BNF, making grammars written using EBNF easier to layout and look good; great if I was working with EBNF. The yacc file input tools I tried (yaccviso, Vyacc) were a bit too fragile and the output was not that good.

Bison has an option to generate a output that can be processed into graphical form (using graphviz as the layout engine). Unfortunately the graphs produced are as visually tangled as the input grammar and if anything harder to follow.

It is possible to produce great looking visual diagrams using a simple tool if you are willing to spend lots of fiddling with the input grammar to control the output. I wanted to take the grammar as written (i.e., a yacc input file) and am willing to accept less than perfect output.

Most of the syntax rules in a yacc grammar are straight forward sequences of tokens that have an obvious one-to-one mapping and there are a few commonly seen idioms. I decided to write a tool that concentrated on untangling the idioms and let the simple stuff look after itself. One idiom that has a visual representation very different from its yacc form is the two productions used to specify an arbitrary long list, e.g., a semicolon separated list of ys is often written as (ok, there might perhaps be times when right recursion is appropriate):

x :  y       | 
     x ";" y ;

and I wanted something that looked like (from the sql-lite web site, which goes one better and allows support for the list to be optional:

Semicolon separated list of stmts.

Graph layout is a complicated business and like everybody else I decided to use graphviz to do the heavy lifting (specifically I would generate the layout directives used by dot). All I had to do was write a yacc grammar to dot translator (and not spend lots of time doing it).

The dot language provides a directives that specify the visual properties of nodes and the connections between them. For instance:

n_0 -> n_3
n_0 -> n_1
n_1 -> n_3
n_1 -> n_2
n_2 -> n_1

is the dot specification of the optional semicolon separated list of sql-stmts displayed above.

Dot takes a list of directives describing the nodes and edges of a graph and makes its own decisions about how to layout the output. It is possible to specify in excruciating detail exactly how to do the layout, but I wanted everything to be automated.

I decided to write the tool in awk because it has great input token handling facilities and I use it often enough to be fluent.

Each grammar rule containing one or more productions is mapped to a single graph. When generating postscript dot puts each graph on a separate page, other output formats appear to loose all but one of the graphs. To make sure each rule fitted on a page I had the text point size depend on the number of productions in a rule, more productions smaller point size. The most common idioms are handled (i.e., list-of and optional construct) with hooks available to handle others. Productions within a rule will often have common token sequences but the current version only checks for matching token sequences at the start of a production and all productions in a rule have to start with the same sequence. Words written all in upper-case are assumed to be language tokens and are converted to lower case and bracketed with quotes. The 300+ lines of conversion tool’s awk source is available for download.

The follow examples are taken from an attempted yacc grammar of C++ done when people still thought such a thing could be created. While the output does have a certain railroad diagram feel to it, the terrain must be very hilly to generate those curvaceous lines.

Unqualified C++ identifiers.

and the run of the mill rules look good, a C++ primary-expression is:
Syntax of C++ primary expressions.

and we can rely on C++ to push syntax rule complexity to the limit, a postfix-expression is:
C++ postfix expression.

What about the idioms? A simple list of items looks good:
List of string literal diagram.

and slightly less good when separators are involved:
List of comma separated assignments.

and if we push our luck things start to look tangled:

With a bit more work invested on merging token sequences common to two or more rules the following might look a lot less cluttered:
Equality expression syntax.

Apart from a few tangled cases the results are not bad for a tool that was a few hours work. I will wait a bit to see if the people I deal with find this visual form of use.

In the meantime I would be interested to hear about my readers experience with visualizing grammars, using dot to this kind of thing and any suggestions they have. As a long time user of dot I know that there are lots of ways of influencing the final layout (e.g., changing the ordering or edges and nodes in its input), I will have to be careful not to get pulled down this rabbit hole.

  1. John Doe
    June 14th, 2012 at 02:51 | #1
  2. June 14th, 2012 at 12:12 | #2

    @John Doe
    Yes, its connected to one of the links in the article.

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