See you there. -m

Did the actual output end up anywhere near where I wanted it? Sadly no: it was worse than silence. So that’s a good lesson for me to keep in mind as I continue to blog.

I appreciate the detailed feedback!

As written, and probably unintentionally, your original post suggests that immutability leads to silent runtime failures, while mutation is easy to follow and understand. This is exactly backwards, and is critical to understanding Clojure.

I hope you will continue to post your experiences, and make the tentative, learning nature of these posts clear in the body of the text as you go. Also, if you haven’t already, give the IRC a try, there is almost always someone there to talk to.

In your Java example, you have a reference (the customer variable) to a mutable object (an instance of Customer). You call methods on that object to modify the state of the object. If you have multiple references to that instance of Customer, all those references see those changes. They may be what you want, but often times, especially in the context of a multithreaded application where multiple threads are trying to update the Customer, that could be a big problem.

In Clojure, instead of modifying the Customer instance that your reference customer points to, you create a new “Customer instance” and modify the reference to point at the new “Customer instance”. I put that in quotes because you will use a Map to store the Customer data, not an object, but either way, it’s the data structure that holds the customer data. This still has the same effect of modifying the Customer and does it in a way where multiple threads could do it concurrently without a problem.

I’ve modified your example to use a ref and you can see that calling add-order twice results in adding two orders to the customer:

http://gist.github.com/242000

Unfortunately, I actually find the Java code more readable in this example as well

Further, you don’t need to use structs — regular maps will do, unless you have a good reason to use structs’ specific talents. e.g.:

(update-in customer [:orders] conj {:rush? true :lines [{:qty 6 :code "TAL"} {:qty 5 :code "HPK", :skippable? true} {:qty 3 :code "LGV"}]})

There’s no way that formatting is going to come through!

It was fun trying to combine (or) and (def) with a macro. I’m learning about macros slowly but don’t understand why one of these works and the other fails: