Let's keep chatting offline. There might be issues I run into that you know how to deal with, or vice versa. Can you send me an email? My address is on my profile.

Edit 16 minutes later: I've gotten Arc running on Windows. Here are the instructions I followed:

0. Install Racket and Git like above.

1. Open Git Bash from the Windows menu and run this command:

    git clone https://github.com/arclanguage/anarki

    (current-directory (find-system-path 'home-dir))
    (current-directory "anarki")
    (load "boot.scm")
    (tl)

A complete and downloadable tutorials on Arc language available at facebook.com/groups/epspost/ > contains installation files and tutorials with a step-by-step instructions for setup and installation for Windows users. Complete downloading 4 Part Files and then use 7zip to extract and combine them in one single folder called "Arc Learning Compilation from EPSpost"

  ; SXML
  (tagname (@ (attr "value") (attr2 "value2"))
    (tagname2)
    (tagname3 "data"))
  
  ; sml.arc's "old format"
  (tagname (@ attr "value" attr2 "value2")
    (tagname2)
    (tagname3 "data"))
  
  ; sml.arc's current example
  (tagname attr "value" attr2 "value2"
    (tagname2)
    (tagname3 "data"))

  ; X-expressions
  (tagname ((attr "value") (attr2 "value2"))
    (tagname2)
    (tagname3 "data"))

As for SXML, I think it's pretty nice that the SXML standard actually has well-defined ways to use doctypes, namespaces, processing instructions, and comments. That makes it more capable of representing an actual XML document. The spec is only a few pages long, and half of that is preamble, so it would be pretty simple to implement it faithfully.

Paul Graham has this to say about hygiene in the tutorial:

  Some people worry unduly about this kind of bug.  It caused the
  Scheme committee to adopt a plan for "hygienic" macros that was
  probably a mistake.  It seems to me that the solution is not to
  encourage the noob illusion that macro calls are function calls.
  People writing macros need to remember that macros live in the land
  of names.  Naturally in the land of names you have to worry about
  using the wrong names, just as in the land of values you have to
  remember not to use the wrong values-- for example, not to use zero
  as a divisor.

  (mac n-of (n expr)
    (w/uniq ga
      `(let ,ga nil
         (repeat ,n (push ,expr ,ga))
         (rev ,ga))))

It seems we don't particularly follow that rule here on the Arc Forum. For instance, a few of us have agreed that a certain behavior in Arc 3.1 is a bug: When we make a function call to a local variable, we don't want a global macro of the same name to take effect, which is what happens in Arc 3.1. If we were keeping locals and globals separate, we wouldn't even encounter this problem.

Which means that if we want to write macros that are hygienic, we can't write them in quite the way we see in arc.arc or the tutorial. If we're dedicated to hygiene, we might even want to rewrite arc.arc to fix its hygiene issues... but that's practically the whole language, so it effectively starts to be a new language project. The Anarki arc2.hygiene branch, Penknife, ar, Semi-Arc, and Nulan are several examples of Arc-based or Arc-inspired projects that pursued some kind of hygiene.

If we don't mind the lack of hygiene in arc.arc but only care about proper hygiene for our own new macros, it is possible to be diligent about hygiene in plain Arc 3.1 or Anarki:

  (mac n-of (n expr)
    (w/uniq ga
      (rep.let ga nil
         (rep.repeat n (rep.push expr ga))
         `(',rev ,ga))))

  (mac n-of (n expr)
    `( ',(fn (n expr)
             (let a nil
               (repeat n (push (expr) a))
               rev.a))
       ,n
       (fn () ,expr)))

  (mac qq-with args
    (let (body . rev-bindings) rev.args
      (let (vars vals) (apply map list (pair rev.rev-bindings))
        `(',list `',(fn ,vars ,body) ,@vals))))
  
  (mac n-of (n expr)
    (qq-with n n expr `(fn () ,expr)
      (let a nil
        (repeat n (push (expr) a))
        rev.a)))

I'm constantly looking for people to hack with as well. Some ideas:

a) We could work on anarki. For example, try out the latest version sometime when you have time and let me know if mktemp broke again on Windows. Help kinnard and me expand the install instructions for Anarki to Windows. Neither of us knows how to run Arc there, so this would be very valuable. These small-ish ideas might be the start of a larger project.

Programming is my life. I like to think I'm a master of it. But I'm completely alone. Also I hate my jobs, I hate big companies, I hate bullshit, I hate TDD, I hate code reviews, I hate estimations; it's a pain man.

Poker, while definitely not as deep as software, is the exact opposite. They do TDD? I take money. They do code reviews? I take money. bullshit? a fountain of money...

The advantages of the language are numerous: raw macro, short names, incredible library, awesome operators, right choice of data structures, etc.

Arc is more like Common Lisp in that you can call whatever you want while expanding a macro, and if you make a mistake you might end up with an infinite regress of macroexpansion or something like that. Its macros are not hygienic which again creates room for certain kinds of bugs, but some of us here tend to think of those as learning experiences ^_^ whose benefits outweigh their pain.

I never said the overloading of "," was accomplished with macros.

The example I gave was of a macro whose name was overloaded in a type-directed way, similarly to the way "," is overloaded in Idris. (Maybe the macro itself is named ",".) My point is that if the macro system is designed to support that kind of overloading, then sometimes a macro call will depend on intermediate results obtained by the typechecker, so we can't run the macroexpander in a phase of its own.

---

For the sake of remembering Idris more accurately, I checked out the Idris docs to look for the particular "macro system" I was dealing with.

It turns out what I mean by the "macro system" in Idris is its system of elaborator extensions (http://docs.idris-lang.org/en/latest/reference/elaborator-re...). The elaborator's purpose is to fill in holes in the program, like the holes that remain when someone calls a function without supplying its implicit arguments.[1]

It's pretty natural to handle name overloading in the same phase as implicit arguments, because it's effectively a special case. Thanks to dependent types and implicit arguments, instead of having two differently typed functions named (,), you could have one function that implicitly takes a boolean:

  (,) : {isType : Bool} ->
    if isType then (... -> Type) else (... -> (a, b))
  (,) = ...

Another "macro system" I encountered in the docs was Idris's system of syntax extensions (http://docs.idris-lang.org/en/latest/tutorial/syntax.html). As far as I can tell, these apply before the elaborator and typechecker, but they might not be able to run arbitrary code or break hygiene. Maybe someday they'll gain those abilities and become a Nulan-like macro system.

I have no experience with Idris or dependent types, so I may be completely wrong.

But from my understanding, a dependent type system allows for types to include values (which are evaluated at compile-time).

If so, then you might not even need macros at all to create the "," behavior.

Instead, you create a typeclass which will dispatch to the appropriate behavior:

  data Pair a b = pair a b

  interface Comma a b c where
    comma : a -> b -> c

  Comma Type Type Type where
    comma = Pair

  Comma a b (Pair a b) where
    comma = pair

  -- Type
  comma Integer Integer

  -- (Pair Integer Integer)
  comma 1 2

  foo : a -> b -> (comma a b)

But I think if your language allows for that, it's inconsistent with the way that macros work.

If you want powerful Arc-style macros, then the macro must be run before any types are known.

Any system which allows for that kind of type-based dispatch is different from a Lisp-style macro system, and is probably less powerful.

For example, you won't be able to create macros which create new variable bindings (like "let", "with", etc.) because the type of the variable is not known until after the macro is run.

I'm not sure what a system like that would look like, or if I would even call it a macro system.

I think it makes more sense to have two separate systems: a macro system that deals with syntax only, and an overloading system that deals with types. That's what Nulan does.

If the macro itself is being referred to by an overloaded name, then the name needs to be resolved before we know which macro to call.

(I'm not sure if macros' names can be overloaded in Idris; I'm just guessing. Actually, I don't know if Idris lets users define overloaded names. I just know a few built-in names like (,) are overloaded.)

---

"If you mean that the primitives should appear to be macros (even though they're implemented with compiler magic), then I agree with you."

Macros deal with things at the syntax level, far earlier than any kind of type overloading.

As far as the macro is concerned, it simply sees the syntax

  (*list [ (*symbol ",") (*integer 1) (*integer 2) ])

Basically, I view type inference/checking/overloading occurring in a phase after macro expansion.

Is there any benefit to mixing the type overloading phase and the macro expansion phase?

----

It occurs to me that you might not be referring to a macro which expands to syntax, but instead using the syntax within the macro body itself.

In Nulan, that isn't a problem either. A macro is essentially a function from Code to Code, so the macro body is compiled/macro expanded/type checked just as if it were a function.

Thus any type overloading will happen inside the macro body, before the macro is ever run.

That does mean that the type checker must be able to overload based solely on the types found within the macro body. In other words, it cannot dynamically dispatch.

----

I don't think it's possible for the primitives to be macros, for the simple reason that the language needs axioms.

It looks like the author forked their existing project Arcadia[1] to make Arc++, and difference has something to do with the memory management strategy (adopting C++'s shared_ptr).

There are a couple of ways I think that can get more complicated:

In languages that resolve overloaded names in a type-directed way, I think the reconciling of static types and macros gets quite a bit more challenging due to the underlying challenge of reconciling name resolution with macros.

For instance, I think Idris has different meanings for the name (,) depending on if it occurs as a value of type (Type -> Type -> Type) or a value of type (a -> b -> (a, b)). In one case it's a constructor of tuple types (a, b), and in another case it's a constructor of tuple values (a, b).

Idris's macro system actually seems to have some special support for invoking the typechecker explicitly from macro code, which I think helps resolve names like those. I haven't actually succeeded in writing macros in Idris yet, and it seems to be a complicated system, so I'm not sure about what I'm saying.

Secondly, one potential goal of a macro system is that all the language's existing primitive syntaxes can turn out to be macros. That way, future versions of the language don't have to inherit all the crufty syntaxes of the versions before; they can stuff those into an optional library. If the language has sophisticated compile-time systems for type inference, term elaboration, name resolution, syntax highlighting, autocompletion, etc., then hopefully the macro system is expressive enough that the built-in syntaxes are indistinguishable from well-written macros.

Arc already gives us things like (macex ...) that can distinguish macros from built-in special forms, so maybe seamlessness isn't a priority in Arc. But if it were, and Arc had static types, we would probably want Arc to have type inference as well, which could complicate the macro system.

I may be wrong on this, but it seems to me that static typing does not prevent macros at all (true or otherwise).

I switched Nulan to use static typing, yet it's using the same kind of macro system that it used when it was dynamically typed.

As far as I can tell, a macro is simply a compile-time function that accepts code and returns code. If so, then its static type is "Code -> Code".

In Nulan, the Code type might be defined like this:

  (TYPE Code
  | (*integer Integer)
  | (*number Number)
  | (*string String)
  | (*symbol String)
  | (*gensym Integer)
  | (*list (List Code)))

Within the macro's body, you can pattern match on the Code, you can map/filter on it just like in Arc, you can dynamically return Code, etc.

In Nulan, this is made easy with the & syntax, which is just a shorthand for the Code type:

  # These two are equivalent
  &1

  (*integer 1)


  # These two are equivalent
  &(foo 1 2)

  (*list [ (*symbol "foo") (*integer 1) (*integer 2) ])


  # These two are equivalent
  &(foo ~a ~b)

  (*list [ (*symbol "foo") a b ])

  # These two are equivalent
  (MATCH foo
  | &(foo 1 2)
      ...)

  (MATCH foo
  | (*list [ (*symbol "foo") (*integer 1) (*integer 2) ])
      ...)


  # These two are equivalent
  (MATCH foo
  | &(foo ~a ~@b)
      ...)

  (MATCH foo
  | (*list [ (*symbol "foo") a @b ])
      ...)

Based on that, I found this way to write aand, which seems to work at http://www.tutorialspoint.com/execute_clojure_online.php:

  (defmacro aand
    ([] true)
    ([x] x)
    ([x & next]
     `(let [~'it ~x]
        (if ~'it (aand ~@next) ~'it))))
  
  (prn (aand "woo" (list it it) (list it it it)))

  (defmacro capture-it
    ([] 'it))
  
  (prn (let [it "woo"] (capture-it)))

  1 * larger - 5/2 * smaller = 0
  2 * larger - 2 * smaller = 12

  arc> (load "lib/math.arc")
  nil
  arc> (gauss-elim '((1 -5/2) (2 -2)) '(0 12))
  (10 4)