Pure DI is Dependency Injection without a DI Container.

TL;DR: the term Pure DI replaces the term Poor Man's DI.

This post essentially proposes a change of terminology. In my book about Dependency Injection (DI), I was careful to explain the principles and patterns of DI in the pure form, without involving DI Containers. Only in Part 4 do you get extensive coverage of various DI Containers, and even here, what you learn is how the DI principles and patterns map to the various containers.

DI is a set of principles and patterns; DI Containers are optional helper libraries.

However, when I wrote the book, I made a mistake (I probably made many, but here, I'll address a single, specific mistake): in the book, DI without DI Containers is called Poor Man's DI. There are reasons for that, but eventually, I've learned that Poor Man's DI is poor terminology (pun intended). The problem is that it sounds slightly derogatory, or at least unattractive; it also doesn't communicate the message that DI without a DI Container is, in many cases, better than DI with a DI Container - on the contrary, it sounds like it's not as good.

Apart from my book, I've attempted to describe the trade-off involved in going from Poor Man's DI to using a DI Container in various articles:

Based on the reactions I've received, it seems like my readers really like their DI Containers. Perhaps they're just afraid of the alternative, because it's called Poor Man's DI.

For these reasons, from now on, I'll retire the term Poor Man's DI, and instead start using the term Pure DI. Pure DI is when you use the DI principles and patterns, but not a DI Container; it's what I've been doing for the last 1½ years, as well as many years before I wrote my book.

P.S. 2018-04-15. Naming is hard. When I came up with the alternative name of Pure DI, I was already in the process of changing my focus to functional programming, but at that time, I'd still not realised how big a role referential transparency plays in strictly functional languages like Haskell, with its emphasis on pure functions.

The purity implied by Pure DI, unfortunately, has nothing to do with purity in the functional sense of the word. In fact, DI makes everything impure.

When I decided on the term Pure DI, I did it because it sounds vaguely like Poor Man's DI, so that it'd be easy to remember. Additionally, I picked the word pure because it can mean essence, and I thought that, in a sense, Pure DI can be viewed as the Platonic ideal of DI. Finally, pure sounds like something desirable, so because of those reasons, I believed that it'd be a good term. People seem to have picked it up, so in that sense I think that I chose the right word, but it can be confusing if viewed through the lens of functional programming terminology.

Pure DI has nothing to do with pure functions.

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Tuesday, 10 June 2014 06:10:00 UTC


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