Wednesday, April 3, 2019

Accessors Have Message Obsession

Just came across and older post by Nat Pryce on Message Obsession, which he describes as the opposite end of a spectrum from Primitive Obsession.

The example is a set of commands for moving a robot:


Although the duplication is annoying, the bigger problem is that there are two things, the verb "move" and a direction argument, mushed together into the message name. And that can cause further problems down the road:
"It’s awkward to work with this kind of interface because you can’t pass around, store or perform calculations on the direction at all."

He argues, convincingly IMHO, that the combined messages should be replaced by a single move: message with a separate direction argument. The current fashion would be to make direction an enum, but he (wisely, IMHO) turns it into a class that can encode different directions:

class Direction {

So far so good. However...

...we have this message obsessions at a massively larger scale with accessors.


Every single attribute of every single class gets its own accessor or accessor pair, again with the action (get/set) mushed together with the name of the attribute to work on. The solution is the same as for the directions in Nat's example: there are only two actual messages, with reified identifiers.
-set:identifier to:value.

These, of course, correspond to the GET and PUT HTTP verbs. Properties, now available in a number of mainstream languages, are supposed to address this issue, but they only really address to 2:1 problem (getter and setter for an attribute). The much bigger N:2 problem (method pair for every attribute) remains unaddressed, and particularly you also cannot pass around, store or perform calculations on the identifier.

And it turns out that passing those identifiers around performing calculations on them is tremendously powerful, even if you don't have language support. Without language support, the interface between the world of reified identifiers and objects can be a bit awkward.

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