Convenience Is not an -ility

Much has been said about the importance and challenges of designing good API's. It's difficult to get right the first time and it's even more difficult to change later. Sort of like raising children. Most experienced programmers have learned that a good API follows a consistent level of abstraction, exhibits consistency and symmetry, and forms the vocabulary for an expressive language. Alas, being aware of the guiding principles does not automatically translate into appropriate behavior. Eating sweets is bad for you.

Instead of preaching from on high, I want to pick on a particular API design 'strategy,' one that I encounter time and again: the argument of convenience. It typically begins with one of the following 'insights:'

• I don't want other classes to have to make two separate calls to do this one thing.
• Why should I make another method if it's almost the same as this method? I'll just add a simple switch.
• See, it's very easy: If the second string parameter ends with ".txt", the method automatically assumes that the first parameter is a file name, so I really don't need two methods.

While well intended, such arguments are prone to decrease the readability of code using the API. A method invocation like

parser.processNodes(text, false);

is virtually meaningless without knowing the implementation or at least consulting the documentation. This method was likely designed for the convenience of the implementer as opposed to the convenience of the caller — "I don't want the caller to have to make two separate calls" translated into "I didn't want to code up two separate methods." There's nothing fundamentally wrong with convenience if it's intended to be the antidote to tediousness, clunkiness, or awkwardness. However, if we think a bit more carefully about it, the antidote to those symptoms is efficiency, consistency, and elegance, not necessarily convenience. APIs are supposed to hide underlying complexity, so we can realistically expect good API design to require some effort. A single large method could certainly be more convenient to write than a well thought-out set of operations, but would it be easier to use?

The metaphor of API as a language can guide us towards better design decisions in these situations. An API should provide an expressive language, which gives the next layer above sufficient vocabulary to ask and answer useful questions. This does not imply it should provide exactly one method, or verb, for each question that may be worth asking. A diverse vocabulary allows us to express subtleties in meaning. For example, we prefer to say run instead of walk(true), even though it could be viewed as essentially the same operation, just executed at different speeds. A consistent and well thought out API vocabulary makes for expressive and easy to understand code in the next layer up. More importantly, a composable vocabulary allows other programmers to use the API in ways you may not have anticipated — a great convenience indeed for the users of the API! Next time you are tempted to lump a few things together into one API method, remember that the English language does not have one word for MakeUpYourRoomBeQuietAndDoYourHomeWork, even though it would seem really convenient for such a frequently requested operation.