[eru]

I've asked a similar question, along the lines 'Can you explain Dependency Injection to me, assuming I already know Haskell?'

And the answer was basically along the lines of: it's a fancy way to pass something like a function argument.

[mypalmike]

It's primarily a technique used in object oriented programming. So it's hard to translate to Haskell.

The big picture of what a DI framework does is let you declare your structural object graph using a config file or decorators and have the whole thing instantiated at runtime automagically.

The detailed view is "a fancy way to pass something like a function argument". An object that has dependencies gets them passed in ("injected") at runtime rather than calling dependent function directly or internally instantiating dependent objects and calling them.

Doing things this way in OO languages has a number of benefits, including improved testability.

[mjr00]

It's more subtle than that. DI and DI frameworks are two separate things that often get conflated.

Injecting dependencies when you instantiate an object, or passing dependencies into a method via arguments, rather than having methods or constructors create their dependencies, is good design and increases testability.

On the other hand, DI frameworks are, IMO, an awful awful mess and mistake and I'm glad the industry has moved away from them. The problem with DI frameworks is that setting parameters automatically is only one part of it; the other part of it is object lifecycle management. After all, a Foo is automatically injected into your class, it means the DI framework needs to know how to create a Foo, and dispose of a Foo.

This is where you get into per-request sessions, context hooks, and all of the stupid Spring bean bullshit everyone has come to hate and is really the main reason for so much anti-"enterprise" software patterns.

Funnily enough, it is unnecessary. Just make your dependencies into parameters and you'll get 95% of the benefit of DI. The last 5% is the code to wire up the object graph at certain entrypoints which can be large, but should be simple code.

[mypalmike]

Agreed. I was mostly trying to give the briefest overview I could for a hn comment.

I don't love spring, but I've seen the benefits from having a well-organized, declarative graph of the top-level objects (i.e. the core objects that live for the entire process lifetime). It provides a clear pattern for how to add new code to a large, growing codebase. Without such a structure, devs end up tacking new code on in arbitrary ways.

[eru]

> It's primarily a technique used in object oriented programming. So it's hard to translate to Haskell.

Yes. I specifically asked like that to avoid getting a description of DI just couched in more OOP jargon but also to provide some alternative programming vocabulary (so you don't have to try to explain everything in everyday English, which seldom goes well).