It's rather a dishonest argument to use an example like that, because it's an artifact of bootstrapping the prototype off MzScheme. A more convincing argument would be strange behavior resulting from the way something was defined in arc.arc.
Is annotate a general mechanism, or specifically for defining macros? Is ellipsize supposed to limit its output to 80 characters or display at most 80 characters of the input? Is median of an even-length list supposed to return the lower of the middle two? Is cdar supposed to be missing? Is (type 3.0) supposed to be an int? Is support for complex numbers supposed to be in Arc? Is client-ip supposed to be there, or is it left over from something? Does afn stand for "anonymous fn"? What does "rfn" stand for?
These are all real questions that I've faced while trying to write documentation. Let me make it clear that these are not rhetorical questions; I'm more interested in getting actual answers than arguing about using the code as the spec.
Wasn't the point keeping the names car and cdr so you can compose them? (I remember reading such in one of pg's essays.) Then it seems to me to take full advantage of that you need to provide those names for use.
I don't think it is unreasonable to do the following, but it is currently not provided in Arc:
I didn't mean Arc will never have cdar. But to avoid having a language designed according to arbitrary rules rather than the actual demands of the task, I've been trying to be disciplined about not adding operators till I need them.
It doesn't matter whether features are deliberate or not. It's very common in math for someone to discover something that has interesting properties they never imagined. In fact, it's probably closer to the truth to say that if a mathematical concept doesn't have properties the discoverer never imagined, it's not a very interesting one.
Lisp itself is an example of this phenomenon. JMC didn't expect to use s-expressions in the real language, but they turned out to be way more useful than he envisioned.
I'm not just splitting hairs here, or trying to defend myself. In design (or math), questions of deliberateness are not binary. I'll often decide on the design of an operator based on what looks elegant in the source, rather than to meet some spec, just as Kelly Johnson used beauty as a heuristic in designing aircraft that flew well.
It's a good argument in general sense, but I doubt it is applicable in library API.
If you're delivering a final product, users don't care if some design is deliberate or not; they care it is good or bad. If you're deriving mathematic proof, others don't care if some choice is deliberate or not; they care if it is correct, beautiful, or useful to prove other theorems.
That's because changing your choice afterwards won't break existing products or proofs that relied on the previous choices.
In the case of library API, changing your choice does break existing software relying on the library. In the current Arc case it is forewarned so it's perfectly fine, but at some point (50years from now, maybe?) you have to let more people write software on it; by that moment it should be clear that what they can rely on and what they cannot.
by that moment it should be clear that what they can rely on and what they cannot
The only difference if the implementation is the spec is how they know what they can rely on. If the implementation is the spec, they decide by reading the source; if it's a document writen in English, they decide by reading that.
Implementation can be, and will be, changed, inevitably. Then does the language change as well, or the languages remains the same but just implementation is improved? How can you tell the difference purely from the source?
Some Scheme implementation evaluates arguments left to right. You can see that by reading the source. In future, it may switch it right to left, maybe for better optimization. The spec in natural language, or more formal and abstract form like in Appendix A of R6RS, can explicitly say the order of evaluation is unspecified. How you tell your users that they should not rely on the evaluation order purely by the source code, given the state of most programming languages?
Ideally I like to think the source only describes the spec and the compiler and runtime figure out the details, so maybe spec-by-source and spec-by-other-notation will converge in future. Is that what you are talking?
(Please don't argue that unspecified evaluation order is bad or not; I just use that example to illustrate the point. And incidentally, since Arc is defined in terms of Scheme, the order of argument evaluation order is just unspecified as well. But it's just delegating the spec to a lower level.)
One point everybody else is missing: since arc explicitly makes no claims of backwards compatibility, the notion of a spec is meaningless.
If the goal of a language is to be readable there's nothing wrong in the implementation being the spec. Consider it a form of self-hosting, or of eating your own dogfood.
An implementation in a reasonably high-level declarative language is a more reasonable 'spec' than one in C. More features are amenable to checking just by reading rather than by execution.
When something is obviously a bug it doesn't matter if it's a bug in the spec or the implementation.
Those two categories -- obvious bugs, and questions about what is or is not in the language that should be answered by reading the code rather than executing it -- seem to cover all the objections people have raised.
At least I'm talking about the attitude of spec-by-source in general, not particularly about Arc, FYI.
Edit: I agree that more abstract, declarative language is closer to spec-by-source. If somebody says Prelude.hs is the spec of Haskell's standard library I agree. But the core language semantics is still not in Haskell itself, is it? (I'm still learning. Correct me if I'm wrong.)
The problem is that as people learn the language they will build mental maps of what works and what doesn't and in the process will write code that depends on things that could legitimately be considered bugs or arbitrary side effects of the current implementation.
Whether or not this matters to you or even should matter is another concern, but this has been a spot of contention for languages like Python and OCaml whose spec is the code.