Roads Less Taken

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Tirade, Supporting Embedded Text

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Two years ago I ended up creating Tirade - a new “file format” for Smalltalkers. Or rather, a way to serialize stuff into a sequence of Smalltalk messages with literals as arguments. I have written a few blog articles about Tirade so I will not go into details in this one.

One thing that has been disturbing with Tirade is that I wanted it to be the main format for serializing Deltas, the new implementation of “21st Century ChangeSets”. This means I want Tirade to handle Smalltalk source code in the best possible way. Ideally I would want the Tirade file to be editable in a text editor if I wanted, and not being broken by that.

So, what properties do we want:

  1. No escaping of special characters. In regular Tirade strings (just like in Smalltalk) need to escape the single quote as doubled single quote, and that would suck for Smalltalk code of course.

  2. No length encoding. One way to avoid escaping is to store the length of the data before the actual data - like a Netstring for example. This prohibits easy editing in a text editor though, since that would change the length.

  3. A reasonable syntax. Tirade so far has been a subset of Smalltalk (disregarding lack of receiver to the left), but I think we might have to break that a bit here.

After pondering this for a while I have come up with this solution which feels kinda nice, but if someone has an even better idea I am all ears. This is how it could look embedding a method source in Tirade:

class: #MyClass selector: #at:put: source: [
    at: pos put: arg
    "Put something here"

    ^array at: pos put: arg

So what gives here?  We are reusing the syntax for Smalltalk blocks without arguments. Simply […content…]. The content will be delivered as a String and the guarantee is that it will be received exactly as sent. There is a trick here - this is what Tirade will do:

  1. Write the starter $[ and then a CR

  2. Before each line in the string (a line being all characters up to and including the next CR or up to end) we insert a TAB. This means that the String begins on the line after the opening $[ and all lines will be prefixed with a TAB.

  3. Then, regardless if the last line ended with a CR or not - we add a CR before the closing $]. This makes sure the closing $] ends up on its own line.

The above trick gives us the ability to detect the end of the string because if a line starts with something else than a TAB then we have reached the end. Thus we do not have to escape the $] inside the string and we still don’t need to do length encoding. We DO however need to make sure all lines begin with a TAB, but if you are editing a Tirade file you should just learn that fact. :)

I am not sure if the above is a good solution, but it is ONE solution and I can’t come up with a better one, unless we would use a really “odd” marker at the end in order to not have to escape it, but that feels “dirty” to me.