I spend all my time with Arc working on the forum, so JSON is more important to me than it might be to someone else who primarily focuses on the language.
I was working on the data export feature, which before just dumped a user's posts and comments as a printed statement, but I wanted it to export JSON because that's more portable. Racket has a really nice JSON parser, unfortunately all the forum data is templated, so I can't just pass it to Racket because it doesn't understand the type. There may be a way to get it to work at that level but I don't know what it is.
So I had to get an old JSON parser for Arc and rewrite a bit of it to be able to generate JSON from forum data. I think it was zck's parser but I'm not entirely sure ATM. It works, but it's an awkward workaround. Any data coming from the forum has to go through one parser, and any other data, such as from a remote API would be going through another (Racket's.)
>Is making it work just a matter of calling the Racket primitive on `rep.template` instead of `template`?
Yes, I had to hunt around to even know that existed, because it's not in the docs.
You can get to it from the "Type operations" link in the table of contents. Of course, you might've been drawn to the "Templates" page instead.
And unfortunately, that documentation describes Anarki's stable branch. Making documentation of similar quality for Anarki's master branch might be quite a bit more work. So even if you were familiar with tagged types already, the knowledge that template instances were a tagged type on Anarki master might be hard to discover.
I suppose a more ideal form of the documentation would describe on the "Templates" page that template instances were a tagged type with a certain representation. It could describe the direct implications of that, but it would also have a link to the "Type operations" page for more background.
>I suppose a more ideal form of the documentation would describe on the "Templates" page that template instances were a tagged type with a certain representation. It could describe the direct implications of that, but it would also have a link to the "Type operations" page for more background.
That would have been tremendously helpful, can we do that? Who actually maintains the documentation, can it be updated?
Perhaps we should maintain the docs only in plain-text and only within the repo, just because of the operational overheads of managing multiple branches. At least that way they won't seem to lie to a newcomer. I'm curious to hear what others think.
I've always implicitly assumed that tables were just an implementation detail for templates, and assumed they were like objects. I think that might be why it never occurred to me to apply `len` on a template, and why a large program like news.arc never ran into this gap.
I have no objection to your approach of making them more like tables. I also have no objection to updating the documentation to stop referring to them as tables. One of those options might be less work than the other :)
> I've always implicitly assumed that tables were just an implementation detail for templates, and assumed they were like objects. I think that might be why it never occurred to me to apply `len` on a template, and why a large program like news.arc never ran into this gap.
Ah, interesting! Here's some background that might help understand why having them act like tables might help. `len` wasn't the first function that I found didn't work with templates; it was just the simplest. I was working on adding teardown functionality to unit-test.arc, and I wanted to look at some of the suites that were created, to see if I was adding tests properly. As the templates end up pretty big (one suite with two tests is about 25 lines), and it was late at night, I figured I'd make it simple on myself, and just look at the keys of the template.
This had nothing to do with the desired "production" code, but only with REPL-hacking introspection.
I want to make them reasonably easy to REPL-hack with; whether they're actually tables or not I don't particularly care right now. The most important table functions are probably len, keys, vals, and maybe maptable/each.
My answer to that question has always been, "it depends." The anti-encapsulation ethos (homoiconicity, using lists where other languages may use objects, the entire compiler fitting in one file and being accessible front-and-center, etc.) means that there's always the ability to peel back another layer of the onion when it becomes relevant.
I think it's a documentation issue. I think I had to search the forums to find out about it when I was playing with JSON interop. Nowhere on the actual template page in the Arc documentation does it tell you this is a thing.
Some things I've only been able to figure out by studying the compiler or arc source code itself. Granted, that's illuminating, but it's also sometimes really annoying.
Templates should just enforce a signature for table fields, but otherwise decompose to tables. I think the issue is that the tables generated by (inst) from a template are annotated as type 'tem when that should (could?) be done with a tag that doesn't actually change the type. Since the template name is passed to the function, you could just build the table with default values without annotating it. You could also delete most of the template functions that just serve as template versions of table functions.
edit: I was also assuming templates type-hinted field values but I don't think they do. Maybe they should?
Also, does anyone else find it a bit odd that a language feature like this is in a lib file rather than being part of the core language?
I disagree with your vision for templates. If you just want something that behaves like tables, why not just use tables? A helper that fills in default values would be pretty easy to write.
Think about the use case of news.arc. There's a list of 'objects' that need to be serialized to disk and unserialized from disk. What should happen if you change the default for a template in code? Should the default update transparently for existing objects? If so, you need some way to distinguish tables that were generated by templates. Which implies something that manages them throughout their life cycle.
>Small languages take it as a mark of pride to move as much as possible into libraries :)
Yeah, I've seen projects that show off the power of a language by doing "x in < 100 lines" that just don't count a remote API call with half a million LOC running on a server or something ;) But with language features like macros and templates that have become ubiquitous, I feel like it's kind of cheating not to just fold them into arc proper.
But that's just me... one thing I've learned being here is that I seem to flow against the culture more than with it, so I can just agree to disagree.
> If you just want something that behaves like tables, why not just use tables?
They are tables, that's what's frustrating. They're tables with metadata. From what I can tell reading earler posts about templates, they used to be something that behaves like tables. Interop between forum data and Racket (and any code where tables are expected) is awkward because that incompatibility has to be worked around, resulting in extra code and extra complexity. Templates need a separate API despite having the same behavior as tables.
>If so, you need some way to distinguish tables that were generated by templates. Which implies something that manages them throughout their life cycle.
Fair enough. But why is it necessary to change their type to do so? Why not make this a feature of tables as a whole, if it's useful? Or tag tables in a way that doesn't change their type, if that's possible?
I understand that tradeoffs have to be made and I'm not trying to be cantankerous or dismiss the value of anyone's work, and no, I couldn't do better myself (yet), which is why I'm commenting on it it rather than making a PR. I'm just wondering if this is the best possible implementation of the concept, given how often I and other people seem to run into issues with it.
Don't worry about sounding dismissive, I totally understand where the questions are coming from.
Tables and objects feel like separate concepts, and they have complementary strengths and weaknesses, and one doesn't subsume the other. To me it seems obvious that if we want to have both, we need them to have different types.
For example, sometimes you want the 'dynamic' ability to set arbitrary keys of metadata on a thing. Sometimes you want the same operation to be an error, by providing a schema. How would a single type do both? No language does so, to my knowledge.
Things should have the same type when they have compatible behavior. When they are incompatible, they shouldn't.
Supporting helpers like len and keys may well still make sense. And as the original story did, this is easy to do.
But in general, having incompatible types easily share functions without sharing too much is an open problem: https://en.wikipedia.org/wiki/Expression_problem A language can easily add a method to many types, or add a new type to many methods. But we don't yet know how to achieve both sides.
And honestly, I think the expression problem isn't important. It doesn't take too much code per method/type. And making it easier just encourages large, bloated codebases.
> ...given how often I and other people seem to run into issues with it.
One thing that might be useful here is a list of issues people have encountered with templates. Maybe we should create a wiki page on GitHub and add to it every time an issue comes up. Then we can have a big-picture view of them and a sense of how many are things people need to learn about Arc, and how many are bugs to be fixed.
I believe Anarki behaves exactly the same as Arc's intent when it comes to templates. The changes that I made here seemed strictly superior to the buggy implementation upstream. But if you disagree you should absolutely feel free to just revert the commits and go back to Arc behavior. I don't use Arc anymore, so my opinions are extremely weakly held, you don't have to bother persuading me. Or, if you have some other specific issue in mind, I'd be happy to be persuaded that I'm wrong.
"But in general, having incompatible types easily share functions without sharing too much is an open problem: https://en.wikipedia.org/wiki/Expression_problem A language can easily add a method to many types, or add a new type to many methods. But we don't yet know how to achieve both sides."
I'm trying to follow, but I think you and I must have different understandings of the expression problem. That article lists several known solutions to the expression problem. The solution Anarki uses is `defextend`.
What do you mean by "sharing too much"?
Is Anarki's `defextend` technique already encouraging a bloated codebase, or is there some other technique you're thinking of that would do that?
Yeah, I suppose you could say the problem is 'solved'. I think of it as a trade-off with costs. We don't know how to achieve zero cost.
For example, I absolutely agree with you that 2 lines per method to extend every table method to some new type constitutes a solution for us. But if we had a thousand such types and a thousand such methods, it may seem like less of a solution. But then `defextend` would be the victim rather than cause of bloat.
Ah, you're imagining us having to write and maintain 1000×1000 individual `defextend` forms someday? Yeah, that does seem like a problem that would not feel solved once we got to it. :-p
I don't think that aspect of the expression problem is solvable in a language design. Instead, it's an ongoing conversation in the community. Sometimes the intent of one feature and the intent of another feature interact, leading people to do a nonzero amount of work to figure out the intent of the two features put together. That work is an essential part of what the community is trying to accomplish together, so it's a cost that can't be eliminated. The intent has to be reflected in the code somewhere, so there will be a nonzero amount of code that serves feature-coordinating purposes.
Regardless, I'm optimistic that although the amount of code will be nonzero, it'll still have a manageable size. To the extent we have any kind of consistency around these feature interaction decisions, those consistent principles can develop into abstractions. The only way we'll have 1000×1000 individual intersections to maintain is if we as a community culture are already maintaining 1,000,000 compelling and distinct justifications for them. :)
I haven't read any more than a few papers on it, and maybe only one of those in depth (which I'll mention below). Mostly I'm going by forum threads, wiki articles, and the design choices certain languages make (like Inform's multimethods and Haskell's type classes).
As far as I understand the history, Philip Wadler's work basically defined the strict parameters of the expression problem and explored solutions for it. Separate compilation and the avoidance of dynamic casts were big deals for Wadler for some reason.
That work was focused on Java, where it's easy to define new classes that implement existing interfaces but impossible to implement new interfaces on existing classes.
The solution I'm most familiar with for Java-style languages is the use of object algebras, as described in Oliveira and Cook's "Extensibility for the Masses: Practical Extensibility with Object Algebras" (https://www.cs.utexas.edu/~wcook/Drafts/2012/ecoop2012.pdf). In this approach, when you extend the system with a new type, you define a new interface with a generic type parameter and a factory method for building that type, and you have that interface inherit all the existing factory methods. So you don't have to solve the unsolvable task of implementing a new interface for an existing class, because you're representing your types as type parameters and methods, not simply as classes.
So I think the main subject of research was how best to represent an extensible program's types and functions in a language like Java where the most obvious choices weren't expressive enough. I think it's more of a "how do we allow extensions to be made at all" problem than a "how do we make all the extensions maintainable" problem.
But then, I've really barely scratched the surface of the research, so I could easily be missing stuff like that.
> ... with language features like macros and templates that have become ubiquitous, I feel like it's kind of cheating not to just fold them into arc proper.
It's totally fine to move something into arc.arc if you want to do that. It's always felt like a non-existent distinction in my mind whether something is under arc.arc or libs/. Is Anarki all language or all standard library? Depends on how you look at it. Why does it matter?
> But that's just me... one thing I've learned being here is that I seem to flow against the culture more than with it, so I can just agree to disagree.
This doesn't feel like a disagreement, more like a language barrier. If I understood better I might know whether I agree or not.
arc> (withs nil 3)
Can't take car of nil
This is a minimal example from something I found in unit-test.arc. It's some macros related to setup code -- if there's no setup, I currently generate something like `(withs nil 3)`. But that errors in Anarki.
At least to me, this is expected. The commit you pointed out above switched the null value to '(). The symbol `nil` still evaluates to (). But the need for evaluation implies that it isn't available in contexts that are not evaluated, such as function arguments or in this particular slot of `withs`.
Like I said, happy to revert it if you don't like it. The whole thing came up because of this conversation: https://github.com/arclanguage/anarki/pull/145#issuecomment-.... The motivation was to simplify the Arc implementation. We already have a nil representation in the underlying Racket; it seems unnecessary to so bend over backwards to switch it to something else.
Yeah, that seems better. I'm still tracking down two test failures, but they're not because of this. I think templates now are of type 'tem, not type 'table.
I tried to make some changes to () instead of nil, and I was not a big fan of how it looked. I found it very unusual that unless quoted, parentheses mean function application. Letting () be the way to write the empty list (and I believe it worked differently quoted from unquoted, but I'm not sure offhand) completely breaks my mental model of how Lisps are parsed.
It's failing because `(type (inst 'foo))` is different in Anarki than Arc. It's a simple change to make it work; I just want to do two things before I stop looking at it:
1. Look deeper into the template inconsistencies. Thanks for the files about this in Anarki.
2. Decide if I want to cut support for Arc, or make this code work in both. This might just involve killing the test, as it's not the _most_ useful test.
Ouch, have the tests for unit-test.arc been failing for the past year? :( :( Very sorry about that. I see the failure now.
I somehow forgot that unit-test.arc has its own tests. Could you post the instructions for running the tests in the Readme? That would also have the salubrious side effect of showing people a way to run a bunch of existing tests.
Once that last one is passing (or maybe even before it's passing), should the top-level tests.arc run these tests too? That way this can be caught not only by Travis CI, but also by people running tests.arc according to the readme.
Anarki isn't really intended to avoid or minimize breaking changes. The unit tests verify only that everything is internally consistent. That boundary around 'internal' should include unit-test.arc, I think.
I _think_ it's some weirdness with the nil/empty list thing. I was getting a case where (str x) resulted in the string "nil", but whatever that object was was not treated as nil, for example in conditionals.
Interesting concept. I feel like I'd be ok with it? I want to say we should bind `nil` to `'()`, so existing code would continue to work, but I might be overindexing on compatability and what I'm used to.
I will admit to not being super sure what the real differences between nil and '() are. Presumably it's more than "what is the human-readable representation of the value that terminates a list/is the false value". But I'm not sure what. Also, is there a difference between the quoted and unquoted version? It feels odd to write () in a repl unquoted -- usually, I expect parens to mean a function or macro call.
I watched this just enough to reassure my initial suspicion that I'd seen this a few months ago. It's certainly in my neighborhood, but there are a lot of words in the first few minutes that mean different things to different people. It seems possible that what he means by obesity is just "running slower than optimal". I don't think that matters much. He talks about waste and excess, but it's actually nice to be able to ball up a paper and start afresh when you're writing a novel or a paper. Waste isn't always a bad thing. Civilizations are in some ways defined by what they waste (https://www.ribbonfarm.com/2012/08/23/waste-creativity-and-g...). So I wish I had a more concrete motivation for what he's aiming towards, so I could assess if the waste he's concerned about is something I'm concerned about.
On one hand I'm glad to see radical ideas like this. As I've struggled to make heap allocations safe and thought about how Rust does it, I've often felt acutely uncomfortable that things have to be as they are. So maybe he's right and pointers are refined sugar that we can thrive without.
But I'm not yet convinced by this particular presentation. Depth lists seem to be basically manually allocated memory that is managed by array indexes rather than addresses. All the benefits derive from them having a consistent lifetime. That gives up a lot of the flexibility of heap pointers! Rather than frame this as, "here's a mechanism that is applicable everywhere," which seems patently false, I'd like to see more of an argument that yes, there are programs you can't write without pointers, but you don't actually need them. From this perspective, Rust's position in https://rust-unofficial.github.io/too-many-lists seems more honest:
"I hate linked lists. With a passion. Linked lists are terrible data structures. Now of course there's several great use cases for a linked list... But all of these cases are super rare... Linked lists are as niche and vague of a data structure as a trie."
(Even this is inadequate. Rust is not just giving up linked lists, it gives up up any data structure that may have two pointers to a single allocation. Doubly linked lists. Trees with a parent pointer. And on and on. Maybe all these data structures are super rare. But it gives me the warm fuzzies to know my language can support them. And I need a stronger argument to give them up.)
The trade-off Mu makes is different: you can have any data structure you want, but refined-sugar will cost you performance to ensure safety, and you'll have to deal with a little additional low-level complexity to juggle two kinds of pointers. I prefer this trade-off to anything else I've seen, but I'm still not quite happy with it. I wish there was something better, or some argument that would persuade me to settle with one of these solutions.
I think if your proposal is always strictly better than not closing, then I support it. Programming in Arc tries to take away the need to read raw html, so we can handle a little extra verbosity in the emitted code in some situations.
It looks like the HTML specification defines this as a "non-void-html-element-start-tag-with-trailing-solidus parse error." The spec says that in this case, "The parser behaves as if the U+002F (/) is not present," but also that "[browsers] may abort the parser at the first parse error that they encounter for which they do not wish to apply the rules described in this specification."
I don't know of any browsers that abort the parsing altogether, so it's still reliable to write the HTML that way.
However, the similarity to XML is actively misleading in this case. When you process that document as HTML, you still get structure like this:
So if you're trying to write a polyglot HTML/XML document, self-closing <p /> tags still probably aren't a great option. Closing the paragraphs explicitly, like so, makes it clearer how the structure will end up:
I think modern HTML does have a reliable common subset with XML. Modern HTML treats <br></br> and <p /> as parse errors, but it treats <br /> and <p></p> as valid. To write HTML/XML polyglot content, you just need to pay attention to whether you're dealing with a void element like "br" or a non-void element like "p".
Incidentally, why use an HTML/XML polyglot at all? There are at least a few situations where it can make sense:
- You're serving it as HTML, but (at least someday) you might want to use an XML-processing tool on it or serve it as XHTML.
- You're trying to serve it as XHTML, but you're worried you'll mess up your server configuration and serve it as HTML by mistake.
- You're confident you can serve it as XHTML today, but you have a backup plan to serve it as HTML if needed. In particular, you're afraid someday your XHTML will be invalid due to a bug in your code, a bug in a browser, an intentional spec violation in a browser (e.g. for security or user privacy), or a backwards-incompatible change in the spec. The XHTML spec dictates that an invalid page won't be displayed at all, so if you end up with invalid XHTML for any of those reasons, your site will be rather unusable until you can implement a fix. If that happens at a time you're not ready to drop everything and look at the bug in depth, then you can make a pretty quick switch to serving it as HTML, and most of the page will display again.
Because of the brittle handling of errors, XHTML still hasn't really gotten off the ground. So it seems like the primary value of the HTML/XML polyglot is to serve a document as HTML but use XML-processing tools on it behind the scenes.
A side note...
In the very early days of XML and XHTML, when people were trying to make their HTML pages as XML-like as possible, many browsers would interpret something like <br/> as an element with the tag name "br/". That's why people got into the habit of putting in a space like <br />. That way those browsers would instead interpret the / as an attribute named "/", which was mostly harmless. Nowadays, the space is pretty much vestigial and you can just write <br/> if you want to.