Earlier quoted context omitted.
How is dependency injection more functional? My personal beef is that most of the time it acts like hidden global dependencies, and the configuration of those dependencies, along with their lifetimes, becomes harder to understand by not being traceable in the source code.
Dependency injection is just passing your dependencies in as constructor arguments rather than as hidden dependencies that the class itself creates and manages. It's equivalent to partial application. An uninstantiated class that follows the dependency injection pattern is equivalent to a family of functions with N+Mk arguments, where Mk is the number of parameters in method k. Upon instantiation by passing construct…
This is all well and good, but you also need a bunch of code that handles resolving those dependencies, which oftentimes ends up being complex and hard to debug and will also cause runtime errors instead of compile time errors, which I find to be more or less unacceptable.
Edit: to elaborate on this, I’ve seen DI frameworks not be used in “enterprise” projects a grand total of zero times. I’ve done DI directly in personal projects and it was fine, but in most cases you don’t get to make that choice.
Just last week, when working on a Java project that’s been around for a decade or so, there were issues after migrating it from Spring to Spring Boot - when compiled through the IDE and with the configuration to allow lazy dependency resolution it would work (too many circular dependencies to change the code instead), but when built within a container by Maven that same exact code and configuration would no longer work and injection would fail.
I’m hoping it’s not one of those weird JDK platform bugs but rather an issue with how the codebase is compiled during the container image build, but the issue is mind boggling. More fun, if you take the .jar that’s built in the IDE and put it in the container, then everything works, otherwise it doesn’t. No compilation warnings, most of the startup is fine, but if you build it in the container, you get a DI runtime error about no lazy resolution being enabled even if you hardcode the setting to be on in Java code: https://docs.spring.io/spring-boot/api/kotlin/spring-boot-pr...
I’ve also seen similar issues before containers, where locally it would run on Jetty and use Tomcat on server environments, leading to everything compiling and working locally but throwing injection errors on the server.
What’s more, it’s not like you can (easily) put a breakpoint on whatever is trying to inject the dependencies - after years of Java and Spring I grow more and more convinced that anything that doesn’t generate code that you can inspect directly (e.g. how you can look at a generated MapStruct mapper implementation) is somewhat user hostile and will complicate things. At least modern Spring Boot is good in that more of the configuration is just code, because otherwise good luck debugging why some XML configuration is acting weird.
In other words, DI can make things more messy due to a bunch of technical factors around how it’s implemented (also good luck reading those stack traces), albeit even in the case of Java something like Dagger feels more sane https://dagger.dev/ despite never really catching on.
Of course, one could say that circular dependencies or configuration issues are project specific, but given enough time and projects you will almost inevitably get those sorts of headaches. So while the theory of DI is nice, you can’t just have the theory without practice.