Having previously blogged about being a fan of Railroad diagrams (here) as a means to communicate language syntax, I have been asked about some of the ways details should be represented. I’ve looked around and not actually found an easy-to-read for ‘newbies’ guide to reading railroad diagrams. A lot of content either focuses on the generator tools or how the representation compares to BNF (Backus Naur Form) – all very distracting. So, I thought as an advocate, I should help address the gap (the documentation with TabAkins tool does a good job of explaining things, but its focus is on the features provided rather than understanding the notation).
Reading Railroad Diagrams
The following table provides all the information to help you interpret the Railroad Diagrams, and create syntax representations.
- If you’re only interested in understanding the notation, read just the first two columns.
- If you want to see examples of how to create the diagram, then the 3rd column will help.
Note: Clicking on the diagrams will result in a bigger version of the image being displayed.
|How to Interpret the Railroad||Railroad Representation||Code using tabakins tool|
|The start and end of an expression are shown with vertical bars. |
The expression is read from left to right following a path defined by the line(s).
|Larger diagrams may need to be read both across and down the page to make it sensibly fit a page like this.|
|We can differentiate between literal values and ‘variables’ by shape. A square shape should denote a variable, and a lozenge represents a literal.|
I have to admit to sometimes getting these the wrong way around, but as long as the notation is used consistently in an expression, it isn’t too critical. In this example, we have replaced mp3monster with the name of a variable called domain-name. So if I set the variable domain-name = mp3monster then I’d read the same result.
|We can make parts of an expression optional. This can be seen by following an alternate path around the optional element. |
In this case, we’ve made the domain-name optional. Assuming domain-name = mp3monster, we could get either:
– This is a mp3monster blog OR
– This is a blog
|We can represent optionality by having the flow split to multiple values. Those values could be either literal values or variables. In this case, we now have several possible names in the blog with the choices being mp3monster, phil-wilkins, cloud-native, or the contents of the variable domain-name. So the expression here could resolve to (assuming domain-name = something-else):|
– This is a mp3monster blog OR
– This is a phil-wilkins blog OR
– This is a cloud-native blog OR
– This is a something-else blog
It is typical for the option most likely to be selected to be the value that is directly in line with the choice. Here that would mean the default would be phil-wilkins
|We can have variations on a choice where we can express the choice as being any of the options (one or more e.g. mp3monster and cloud-native) or all of the choices. These scenarios are differentiated by the additional symbols before and after the choice.|
|We can represent the looping with the inclusion of an additional literal(s) or variable(s) by having a second line from the right (exit) of a literal or variable and flowing back into the left (entry) of a literal or variable. Then in the loop of the flow below are the variable(s) or literal(s) that go around between each occurrence of the loop. If our variable was a list now i.e. domain-name = [‘ mp3monster’, ‘cloud-native’] then this would resolve to :|
This is a mp3monster and cloud-native blog
|We can group literals and variables together with a label. These groupings are denoted with a dashed line and usually include some sort of label. In our example, we’ve grouped two literal values together and called that group the blog name.|
All of these constructs can be combined so you can do things like making choices optional, and iterate over multiple variables and literals.
Tips & Tricks to Consider
|Tips||Railroad Representation||Code using tabakins tool|
|Sometimes the number of options in a choice can get impractically large to show in a Railroad diagram. One way to overcome this is to show the first and last options and an ellipsis.|
We can then wrap it with a comment that directs the reader to the complete list of choices.
This way the diagram continues to be readable – the most valuable part of the diagram and easy to locate the specific fine details.
Terminal(‘ is ‘),
Terminal(‘ a ‘),
‘ … ‘,
‘ cloud-native ‘), [‘ See Section X for full list’]),
Terminal(‘ blog ‘))
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