Is static type checking important in software for a bank?

(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at:


One attitude:

I always thought critical applications like banking needed compile time type checking. Clojure is neat and all, but it’s dynamic.

The counter-argument:

They need quality, trustworthy software. Compile time type checking is a tool that can be used to help get there (but how useful it is in getting there depends on how robust the type system is, and Java’s is not particularly robust).

Clear and concise code that is readily understood, avoids visual noise so that the programmer can focus on function, also is a tool that can help with that.

Clojure focuses more on the latter than the former, but that doesn’t necessarily make it worse, and may make it better.

But consider the problems of Java as a reason to give up on Object Oriented Programming, and recall that Object Oriented Programming and static type checking are 2 different things:

Firstly Java encourages lots of modelling. “Let’s sit down with all the power of OO and create of lots of classes that attempt to model the problem at hand. We won’t use UML but instead we’ll evolve it in an agile way.” The model in the investment banking world is actually quite complicated so this OO graph grows quite large. Abstraction is brought in do deal with variance; abstract classes and interfaces proliferate. The type system encourages this. I had used some dynamical languages before and it was quite obvious that we were essentially forcing lots of schema and type definition on to a problem domain that just didn’t want or need it. Sure you can minimize the pain with patterns such as composition over inheritance, better abstractions etc, but we had already admitted to ourselves in some quarters that we were fighting the wrong battle.

And soon it becomes like wading in cement. Our system was predominately conceived around 2007 when it was the heyday of using Test Driven Development (TDD) and to have a heavy reliance on dependency-injection frameworks like Spring. I should state now that I don’t actually dislike Spring with any religious fervour – it just looks redundant in retrospect now that I’m working 95% with FP code. Our app back then also had lots of Mock framework usage – we had both JMock and Mockito flavours. Before our big move to Clojure I’d already decided I just didn’t get on with mock frameworks. They slow down development as you constantly find yourself fighting against lots of incomprehensible tests written dogmatically that rarely test anything useful. I really can’t really think of many situations right now when using Mocking would be a good idea, even in a Java/OO setting.

I had also picked up a consensus view amongst industry peers that the only way to truly break up a big monolithic system like ours was to enforce hardline decoupling around service boundaries. The best way to achieve this is naturally to use different languages. If we’d have stuck with Java we’d never have ‘gone for broke’ to drop a crowbar into our system and to wrestle it apart. Now we’re using Clojure this is essentially what we’re doing, and not in a bad way.