May 31, 2016

...Learn TDD with Codemanship

Squaring The Circle of "Tell, Don't Ask" vs. Test As Close To Implementation As Possible

One of the toughest circles to square in a test-driven approach to designing software is how to achieve loosely-coupled modules while keeping test assertions as close to the code they apply to as possible, so that they are: a. better at pinpointing failures, b. run faster, and c. can travel wherever the code they exercise travels.

The interaction-focused approach to TDD that allows us to write unit tests that are more about the telling and less about the asking (Tell, Don't Ask) can indeed leead to designs that are more effectively modular. But, as Martin Fowler also mentions in his post on Tell, Don't ask, it can lead to developers becoming somewhat ideological "getter eradicators".

These example classes from a community video library system break our Tell, Don't Ask principle by exposing internal data through getters.

But is, per se, by itself a bad thing? Remember that our goal is to share as little information about objects among the classes in our system. So our focus should be on clients who use Title and Member, and what they know about them.

Library, in this example, isn't bound to the implementation classes that have the getters. It only knows about the interfaces Copyable and Rewardable, which only include the methods Library uses. (See the Interface Segregation Principle).

The tests for Library know nothing of Member or Title.

And the assertions about the work that Member and Title do are isolated in test classes that are specifically about those implementations.

In practice, this turns out to be preferable to the favoured Tell, Don't Ask approach in TDD that puts all the assertions about the work in acceptance tests and relies mostly on interaction testing at the unit level. Just because Library told Copyable to register one copy, that doesn't mean Title actually registered one copy. Someone somewhere has to ask that question. Ask. And to ask that question, the number of copies has to be accessible somehow. Of course, we could cheat, and preserve our getter-less implementation by accessing state by the back door (e.g., going direct to a database), but this is far worse than using a getter, IMHO. We're sharing the knowledge, but we're pushing that dependency out of the code where our compiler can see it.

Just as functional programming's "dirty little secret" is that - somewhere, somehow - state changes, Tell, Don't Ask's is that eventually we have to ask about the internal state of objects (even if we're not going to those classes in our code to do it). It may be for use in an acceptance test, or in the user interface, or in a report - but if we don't make object state accessible somehow, then our OO applications can have no O in its I/O.

Having said all that, I do find that Tell, Don't Ask and interaction tests can lead us to more decoupled designs, in that they can lead us to minimal interfaces through which object collaborations happen.

One last though on this; if the only reason we have methods for accessing state is for testing implementations (which, as mentioned, is not really the case, but let's run with it for the sake of argument), greater information hiding could be achieved by internalising those assertions.

This would satisfy both needs of having the work tested as close to where it's done as possible, and exposing as little internal state as possible. Our tests would not actually ask any questions, they would simply be drivers that exercise our objects with various inputs.

Posted 1 year, 5 months ago on May 31, 2016