Fluent events

A few weeks ago me and a colleague were writing a tool that reads a data file (an abstract database description) and executes some sql insertion code for each part of the file. As eager dogfood consumers we chose to use ngr as the concrete syntax for the file.

We anticipated the data file to be very large, so we got ourselves a story telling that we should be able to stream selected parts of it while building elements from others.

So I modified the ngrease parser to produce parse events and the element builder to consume them.

Next I started to make the interface easy to use from the database tool. I wanted to avoid the most difficult and laborous part of stream handling, namely state management. This, combined with the need to delegate parts of the stream to the existing element builder, lead to a solution where every listener method returns a listener that will handle the next event.

After some more coding I even split the listener interface into several interfaces, roughly one for each production rule of the grammar.

At that point I experienced deja vu: a set of interfaces whose methods all return an interface from the set... I've written a fluent interface!

Usually fluent interfaces are written for human users, but this had been written for other programs. Well, of course I had to try the interface with Eclipse and its ctrl-space, and while there are better examples of fluent interfaces around, it certainly felt fluent enough for producing a stream of elements by hand.