Specifications are ultimately defined by their implementations

May 28, 2017

In theory, the literal text of a specification is the final authority on defining what the specification means and requires. In practice, it generally doesn't work out this way; once a specification gets adopted, it ultimately becomes defined by its implementations. Regardless of what the actual text says, if everyone, or most people, or just dominant implementations do something or have some (mis-)interpretation of the specification, those things become the specification in practice. If your implementation doesn't conform to the wrong things that other implementations do, you can expect to have problems interoperating with those other implementations, and they almost always have more practical power than you do. You can appeal to the specification all you want, but it's not going to get you anywhere. People actually using the implementations generally care most that they interoperate, and they don't really care about why they do or don't. A new implementation that refuses to interoperate may or may not be 'correct' by the specification (many people are not well placed to know for sure), but it certainly isn't very useful to most people and it's not likely to get many users in the absence of other factors.

(Of course there can always be other factors. It's sometimes possible to give people no choice about using a particular (new) implementation or very strongly tilt them towards it, and if you do this with a big enough pool of people, your new implementation can rapidly become a dominant one. The browser wars in the late 90s are one example of this effect in action, as are browser engines on mobile platforms today.)

One corollary of this is that it's quite important to write a clear and good specification. Such a specification maximizes the chances that all implementations will do the same thing and that what they do will match what you wrote. Conversely, the more confusing and awkward the specification, the more initial chaos there will be in implementations and the more random and divergent from your intentions the eventual actual in-practice 'standard' is likely to be.

(If your specification is successful, enough of the various people involved will wind up implementing some common behavior so they can interoperate. This behavior does not necessarily have much relationship to what you intended; instead it's likely to be based on some combination of common misunderstandings, early implementations that set the stage for everyone else to copy, and what people settled on as the most useful way to behave.)

(I've sort of written about this before in the context of programming language specifications.)


Comments on this page:

For VHDL, a specification that I've spent far too much time with, I guess this is true with respect to default configurations, mostly because the original definition was unworkable. Elsewhere, possibly the reverse is true, with various "conveniences" and differing interpretations being dropped, possibly at the behest of some of the bigger electronics companies.

Written on 28 May 2017.
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Last modified: Sun May 28 00:18:02 2017
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