I finally got to spend a longer while with Pedestal. It is a pure Clojure web framework, where by pure I mean that you’ll see Clojure on back- as well as front end. For starters I only focused on the front end and that’s what this post is about.
What Is Pedestal?
The documentation is good, but it only explains the fundamental concepts. I think it lacks practical examples and more exhaustive API docs (like inputs and expected outputs of each function, options for each configuration etc.). Or maybe it lacks a tutorial (if I win some time on a lottery, I might write one). There are two good sample applications in their repository: helloworld and chat. Unfortunately, helloworld is very simplistic, and chat is a little bit too complicated for beginners. It does a great job showing off all of the features, but it was a little bit too much for me.
So, after a good while of reading the documentation and reverse engineering the samples, I think I’m finally starting to understand what Pedestal is all about.
On the client side Pedestal is a purely functional, reactive, message-oriented framework. All the state is defined in two pieces: the data model and application model. On the surface each of those models is just a single Clojure map, effectively a tree.
Most of the time you don’t operate on the state directly. You define functions (transforms) that react to messages on specific topics and get current state of data model as argument. They return the new value of the state, and the framework will make it the new data model. Another essential kind of function is emitter: In response to certain messages it emits commands (deltas) to update the application model (like “enable transform”, “disable transform”, “create node”, “set value of node”). Finally, in the application level, you can define functions that change something in the behavior or in DOM in response to those deltas.
Everything goes through this pipeline. If an event happens in GUI, or anywhere in the application, it appends a message to the queue and kicks off all the transforms that bubble up from data model through application model to DOM.
This is a simplistic view that doesn’t do justice to all the features, but I think it’s a good starting point.
Better Sample App for Beginners
During this exploration I wrote another sample application. It’s yet another reincarnation of the TODO app that has a single text entry and a list of everything that has been entered so far. It’s much simpler than the chat demo, and it has plenty of comments explaining what’s going on. This is the kind of application that would help me tremendously during in the learning process, and I hope it saves someone some trouble. You can find the source at Github.