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1 point by soegaard 4018 days ago | link | parent | on: Arc 3.1 (works on latest MzScheme)

See Flatt's blog post:

Further discussions can be found in the mail archive.


1 point by soegaard 4502 days ago | link | parent | on: Asynchronus I/O in arc

Most Scheme implementations doesn't use OS threads due to the cost of context switches. It is standard practice to use one thread for each connection.

See for example this very well written introduction to Systems Programming with PLT Scheme:


3 points by soegaard 4502 days ago | link | parent | on: Asynchronus I/O in arc

MzScheme does not use OS threads. A long time ago it did, but it is pretty hard to make it work on Windows, OS X and Linux at the same time. Also context switching becomes cheaper without OS threads.

If you have found anything on OS threads in "Inside MzScheme" it must be in the section on how to embed MzScheme into a C program (with its own thread).


4 points by soegaard 4531 days ago | link | parent | on: Will Arc ever be as fast as CL?

Implementing call/cc efficiently has been well-reasearched in the Scheme community. For a very well-written account of a non-stack-copying implementation see

R. Kent Dybvig. "Three Implementation Models for Scheme". PhD. Thesis.

Then continue at ReadScheme at "Compiler Technology/Implementation Techniques and Optimization" to see further developments (Look especially for Clinger's papers).


3 points by soegaard 4538 days ago | link | parent | on: Issues Regarding Lisp Adoption

It is actually easy to underestimate DrScheme. It's not just for beginners, but also for wizards.


1 point by tokipin 4538 days ago | link

i'm sure it is very good, i was just talking about the graphical aspects of it. maybe you're referring to my mention of Vim, but i wasn't comparing. Vim is IDE-wise completely lacking, but that's something that doesn't bother me, especially due to its superior-to-all-others editing model. and the reason i say Eclipse would be a better choice is just because it's substantially prettier and more familiar


3 points by soegaard 4538 days ago | link

For editing text Vi and Emacs are preferred by quite a few hackers.

However for editing Scheme, DrScheme is the best choice. Not because it is "graphical", but simply because it was designed with one purpose only: writing Scheme code.

It has all the standard stuff (such as: to show the documentation on an identifier just press F1), but it also contain the Scheme specific stuff, that you won't notice until you try some advanced Scheming.

Among the most impressive features is how precise the error messages are reported. This goes for both standard errors, but not the least when macros are involved. If you compare the error reports you get from a few errorneous macros in DrScheme and in a standard Scheme implementation, you'll see that you can save a lot of time if you use DrScheme.

And while at, don't forget try the macro stepper...


3 points by cooldude127 4538 days ago | link

i like drscheme in general. there is one problem with it that made it simply unusable by me. when you hit run in drscheme to reevaluate your definitions, it obliterates my entire history in the REPL. i need to have that stuff around to retry expressions.


3 points by soegaard 4538 days ago | link

That's by design. [I can dig up a reference, if you are interested]

If you want to retry expressions, stay in the REPL and use ctrl-p (one or more times) to recall previous expressions.


5 points by soegaard 4549 days ago | link | parent | on: Modules : a proposition

Matthew Flatt.

"Composable and Compilable Macros: You Want it When?".

International Conference on Functional Programming


And you are right, making modules and macros work together turns out to be harder than most people realize. Schemers have thought long and hard about the problem. It would be wise to look at the papers at
before designing a module system.


I recommend

"Syntactic Abstraction: the syntax-case expander" by Kent Dybvig. It appeared in the book "Beautiful Code"


2 points by soegaard 4550 days ago | link | parent | on: Eval Inquirry

Hash tabels ought to be easy to fix by setting the MzScheme parameter print-hash-table to #t. See


  "What I don't understand is why you say it's informally-  
  specified. Isn't its specification the source itself, ..."
That's just a roundabout way of saying there is no spec ;-)


2 points by partdavid 4556 days ago | link

I don't think that it is. It's a way of saying that the Arc language satisfies two constraints: it is suitable for writing programs, and suitable for writing specifications. For "exploratory programming" this might be the right thing.

I don't know if Arc actually satisfies that, but I can certainly see the point. Part of this would be a lack of optimization in implementation: implement something in the most straightforward manner, and it is its own specification.


2 points by cadaver 4556 days ago | link

On the other hand, in a separate english-language specification you could say that a certain thing is unspecified and leave it up to the implementation to decide on a certain behaviour. I have been reading a bit in the scheme report (that's a specification right?) and there are a certain number of unspecifications in there.


1 point by cooldude127 4546 days ago | link

i don't arc is intended to have more than one implementation, or at least more than one popular implementation. if this is the case, nothing is unspecified. whatever the implementation does, that is the language.


0 points by soegaard 4556 days ago | link

Letting the implementation be the spec rules out bugs in the implementation. If, say, (+ 1 1) returns 3, then it isn't a bug, since that's what the spec says.


6 points by pg 4556 days ago | link

It means you've specified a bad language.


2 points by soegaard 4555 days ago | link

Sure. In general it isn't so easy to figure out, whether something was done on purpose.

Example: Was it intensional that (1 . + . 1) evaluated to 2?


4 points by pg 4555 days ago | link

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.


9 points by kens 4555 days ago | link

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.


7 points by pg 4555 days ago | link

The former; the latter; yes; yes; yes (for now); yes; the former (for now); anaphoric; recursive.


6 points by parenthesis 4555 days ago | link

I've matched pg's replies up with the questions, to make this discussion easier to read:

annotate is a general mechanism

ellipsize is supposed to display at most 80 characters of the input

median of an even-length list is supposed to return the lower of the middle two

cdar is supposed to be missing

(type 3.0) is supposed to be an int (for now)

support for complex numbers is supposed to be in Arc

client-ip is supposed to be there (for now)

afn stands for "anaphoric function"

rfn stands for "recursive function"


2 points by drewc 4546 days ago | link

But pg's was so much more concise! ;)


4 points by eds 4555 days ago | link

>> Is cdar supposed to be missing?

> yes

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:

arc> (cdar '((1 2 3) (4 5 6))) Error: "reference to undefined identifier: _cdar"

Maybe this is just me missing CL's four levels of composition of car and cdr.


8 points by pg 4555 days ago | link

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.


5 points by parenthesis 4555 days ago | link

I suppose you can cdr:car .

On the one hand, it does feel like all the c....r s should be there.

On the other hand, I think cadr is pretty much the only one I ever actually use; and it is there.


2 points by drcode 4555 days ago | link

I noticed this contradiction, too... :) If we're not going to use c[ad]r composability, why not just use unique, easily distinguishable names for all of these that don't compose:

  car  --> hd
  cdr  --> tl
  caar --> inner
  cddr --> skip2
  cadr --> second
...or something like that. Unique names would reduce errors.


4 points by soegaard 4555 days ago | link

Never mind the example. What troubles me with the the-code-is-the-spec approach, is that for an outsider, it is impossible to tell which decisions where made deliberately and which were accidental.

Just for the record, I find it is fair game to say there is no specification, while the experimentation phase is still going on.


5 points by pg 4555 days ago | link

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.


1 point by shiro 4555 days ago | link

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.


2 points by pg 4555 days ago | link

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.


4 points by shiro 4555 days ago | link

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.)


1 point by soegaard 4554 days ago | link

Actually PLT Scheme guarantees left-to-right order, but that doesn't change your point.


2 points by akkartik 4555 days ago | link

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.


1 point by shiro 4555 days ago | link

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.)


1 point by almkglor 4555 days ago | link

Right. And nobody needs comments in source code, ever.


1 point by akkartik 4555 days ago | link


FWIW, here's how I think comments in source code can be improved, especially in exploratory programming:


1 point by almkglor 4554 days ago | link

Oh come on. What are comments but prose descriptions of the code?

Anyway please reparse my post with <sarcastic>...</sarcastic> around it.


1 point by akkartik 4554 days ago | link

I did get the sarcasm, but not how you extrapolate from my comment.

"There's nothing wrong with a code spec in this case" != "prose specs suck"


1 point by almkglor 4554 days ago | link

Hmm. Must be a bug in my reader ^^


2 points by oconnor0 4555 days ago | link

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.


Arc uses the PLT Scheme reader. The PLT Scheme reader has a non-standard (as compared to RnRS) extension, namely the double-dot notation. The double-dot notation is used to write "infix expressions".


  (x . < . y)  is turned into  (< x y)
  (integer? boolean? . -> . void?) is turned into (-> integer? boolean? void?)
This reader extension can be turned off, but setting the appropriate parameter.