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This is an unfinished project that I'm no longer working on; it is archived here.


ar is now loading and running code from strings.arc. Much of Arc remains unimplemented (see the todo below for the known list).


If you have a file named "foo.arc" in your current directory, you can run it with:

/path/to/ar/run arc foo

(you can type either "foo" or "foo.arc"; files without an extension default to ".arc"). Specifying "arc" loads arc.arc, which in turn loads the Arc compiler ac and the Arc runtime ar.

If you begin foo.arc with a use form like this:

(use arc)

then you don't need to specify "arc" on the command line:

/path/to/ar/run foo

If you have the ar directory on your system path, you can just type "run":

run foo

You can load your "foo.arc" and then go into the REPL with:

run foo repl

or just run the Arc REPL by itself:

run repl

if you have rlwrap:

rlwrap -q \" run foo repl

By default, the use path where files such as "foo" is looked for contains the current directory and the ar directory. To add a local directory to the use path, use a trailing "/". This will run /path/to/my/libs/foo.arc:

run /path/to/my/libs/ foo

You can pass command line arguments into your program by putting them after "--":

run /path/to/my/code/ mylibrary myprogram -- arg1 arg2

To add a remote git repository to your use path, specify the "Git Read-Only" repository URL, such as found on github:

github git url

$ run git:// hello
Cloning into master...
hello, this is hello.arc

Here, has a file, which contains (prn "hello, this is hello.arc").

You can specify a particular commit, tag, or branch with "!":


Using a "git:" URL will perform a "git clone" for you if this is the first time you've used that repository. To avoid having remote libraries code change randomly on you, ar doesn't perform a "git pull" automatically: after the first clone the fetched files will stay the same until you update them youself.

You can do a git-pull to update the repository to the latest revision:

run git repl
arc> (git-pull "git://")

Within Arc, a (use ...) form loads the items in the same way as if they were specified on the "run" command line.

$ run repl
arc> (use "git://" hello)
hello, this is hello.arc

Source code files are loaded only once, whether specified on the "run" command line or in a use form. Thus an Arc source code file can start with a (use ...) as a simple way to load its dependencies.

(However, if you specify a source code file in two different ways such as by /mycode/ foo and /mycode/foo, it will get loaded twice. It might be better to check for whether code has already been loaded by the absolute path of the source file... but I'm still trying to figure out what to do about using or overriding code by their symbolic name).

Due to limitations of the current implementation, using a "git:..." repository will load Arc, git, and use-git into the current runtime. Thus you can't use the remote repository feature with non-Arc languages yet.

Code loaded with "use" is loaded directly into the current runtime with Arc's load, with no separation or isolation between your code and the library's code. One way of providing isolation (somewhat similar to what modules or namespaces supply in other language) is to load code into a different runtime, as described below.


With script, you can write a shell script in Arc. (Though still todo is conveniently accessing the command line arguments).

For example, if the file "hello" contained:

(use arc)
(prn "hello there")

you could run this script with:

$ chmod +x hello
$ ./hello

if you have ar on your path, you can also use env to avoid hard coding the path to ar:

#!/usr/bin/env script
(use arc)
(prn "hello there")

Run tests with:



  • Make Arc (even more!) hackable, enabling people to create their own personal programming language -- beyond what can be done just with using macros.

  • Provide a complete implementation of Arc 3.1, as one of the available languages based on ar.

  • Be at least as good as Arc 3.1 at running a production website; thus for example you should be able to run a news.arc site on top of ar if you wanted to.

  • Use the latest Racket version directly, instead of relying on the "mzscheme" backwards compatibility mode.

  • Fix bugs and make enhancements in the runtime which are easier to do with a compiler which isn't quite as tightly bound to Scheme.

  • And, ideally, to provide a clean and well-factored implementation of all of the above.


  • It's not intended to be within the scope of the ar project itself to come up with some better language than Arc 3.1; though it is the job of ar to support the creation of languages better than Arc 3.1.

    Thus questions such as "would it be better get the standard output port with stdout or (stdout)?" or "should the table constructor function be called table or hash?" aren't for ar to decide; though ar would ideally let you use stdout or (stdout), or table or hash, as you wanted to.

    Of course, the line between the two is rather blurry (at what point does "supporting a better language" become simply "a better language"), and the current ar arguably already crosses the line with changes such as stdout... but to frame the discussion, a question to ask is not "should Arc do X instead of Y?", but instead to ask, "I want to do X. How can we get ar to let me to do that easily?"

  • It's also not a goal of ar to provide a comprehensive implementation of all the features included with ar.

    For example, ar has tests, and so includes a small testing library to run the tests. But it isn't a goal of ar to provide a general purpose testing library; instead, there's just enough of a test framework to run ar's tests.

    As another example, some useful utilities such as ret and cwd are included in ar. But it's not a goal of ar to provide useful utilities; the ones that are included are included because they're needed to load libraries.

    In summary, if it can be loaded as a library, then it doesn't need to be in ar itself. (Since there are some things that we'll be able to move out of ar and into libraries when the loader is further along, ar may be able to get smaller over time).


  • split arc.arc into smaller pieces.

  • allow special forms such as "fn" to be used as a lexical variable, and to be overridden by a macro.

  • tidier implementations of dynamic and implicit.

(see for the above)

  • Let external libraries loaded by "use" declare their default dependencies, but provide some way of overriding them.

  • Isolate the "use" loader so that it can be used with non-Arc languages.

  • Come up with some better name than "dlet" so that we can consistently use "implicit" to refer to parameter based variables.

  • Some form of reification for global variables might provide a more useful import mechanism.

  • Creating new runtimes is slow because all the work of compiling code (such as arc.arc) is done over again every time. If we extracted out the non-functional parts from ac, memoizing the remaining functional parts might help.

  • When a runtime is passed to eval, it isn't running ac from the passed runtime.

  • Date/time tests are failing on different computers... maybe a timezone problem?

  • Optimizations (such as direct-calls)

  • See if we can move coerce and + into Arc.

  • (err "foo" '(1 2 3)) prints "Error: foo {1 2 3 . nil}"

  • defrule is a fun hack but awkward to use for ssyntax: we have to put defrule's in a particular order to specify ssyntax precedence

  • would be nice if typing ^C returned to the REPL

  • ac-nameit, ac-dbname

  • atstrings

  • ac-binaries

  • macex1

  • explicit-flush

  • declare

  • primitives

    • current-process-milliseconds
    • current-gc-milliseconds
    • memory
    • sin
    • cos
    • tan
    • asin
    • acos
    • atan
    • log
  • Arc 3.1 calls ac-macex in ac-assignn... I wonder why?

  • need tests for

    • atomic
    • threads
    • whilet
    • awhen
    • whiler
    • consif
    • check
    • reinsert-sorted and insortnew
    • memo and defmemo
    • prall, prs
    • templates
    • cache, defcache
    • until
    • queue
    • flushout
    • noisy-each
    • trav
    • hooks
    • out
    • get
    • evtil
    • rand-key
    • ratio


  • Arc lists are implemented using Racket's mutable pairs (mpair's)

    as one way to fix the queue bug, and to avoid needing to use pointer operations to modify Racket's "immutable" pairs.

  • quasiquotation is implemented with Alan Bawden's algorithm

    as a fix for list splicing in nested quasiquotes, which was giving people trouble writing macro-defining macros.

  • Function rest arguments are 'nil terminated Arc lists

       (cdr ((fn args args) 1)) => nil
  • the Arc compiler is reflected into Arc

    where it can be hacked in Arc by redefining or extending the functions which implement the compiler:

       arc> (ac-literal? 123)
       arc> (eval 123)
       arc> =
       arc> (ac-literal? =)
       arc> (eval =)
       Error: Bad object in expression #(tagged mac #<procedure>)
       arc> (defrule ac-literal? (isa x 'mac) t)
       arc> (ac-literal? =)
       arc> (eval =)
  • lexical identifiers take precedence over macros

       arc> (mac achtung (x) `(+ ,x 2))
       arc> (let achtung [+ _ 5] (achtung 0))
  • quote passes its value unchanged through the compiler, instead of copying it

    This isn't noticeable when just using quote to quote literal values in the usual way like '(a b c); because the original value isn't accessible to the program we can't tell if it was copied or not.

    However the behavior of quote is visible when using macros, since they can insert arbitrary values inside the quote expression.

    Choosing not to copy the quoted value means we can define inline like this:

       (mac inline (x)
         `',(eval x))

    and we'll get the same value out of inline that we put in:

       arc> (= x '(a b c))
       (a b c)
       arc> (is x (inline x))

    I'm not sure if I understand all the ramifications of this change; but that we can define inline so simply is at least suggestive that this may be the right axiomatic approach.

  • function values are considered literals by the compiler

    This is another change which isn't visible unless you're using macros (there otherwise isn't a way to insert a function value into the source code the compiler compiles).

    In Arc 3.1, a function value can be included in a macro expansion, but it needs to be quoted:

       (mac evens (xs) `(',keep even ,xs))
       (def foo () (evens '(1 2 3 4 5 6 7 8)))
       (wipe keep)
       arc> (foo)
       (2 4 6 8)

    With this change, the function value no longer needs to be quoted:

       (mac evens (xs) `(,keep even ,xs))
  • macro values can also be included in a macro expansion

       (mac bar () `(prn "hi, this is bar"))
       (mac foo () `(,bar))
       arc> (foo)
       hi, this is bar
  • join can accept a non-list as its last argument

       (join '(1 2) 3) => (1 2 . 3)

    which turns out to be useful in macros and other code which works with dotted lists. It means that any list can be split on any cdr, and applying join to the pieces will result in the original list.

  • global variables are represented in Racket's namespace with their plain name

    In Arc 3.1, global variable are stored in Racket's namespace with a "_" prefix, which can be seen e.g. in some error messages:

       arc> x
       Error: "reference to undefined identifier: _x"

    This implementation uses the plain variable name with no prefix:

       arc> x
       Error: reference to undefined identifier: x

    To avoid clashes with Racket identifiers which need to be in the namespace, Racket identifiers are prefixed with "racket-".

  • implicit variables

    which can help make programs more concise when the same variable doesn't need to be threaded through many layers of function calls.

  • implements stdin, stdout, stderr as implicit variables

    removing an unnecessary layer of parentheses.

  • uniq implemented using Racket's gensym

  • readline accepts CR-LF line endings

    which is useful for Internet protocols such as HTTP.

  • [...] is implemented with a macro

    [a b c] is expanded by the reader into (square-bracket a b c). Meanwhile there's a square-bracket macro:

       (mac square-bracket body
         `(fn (_) (,@body)))

    this makes it easier to hack the square bracket syntax.

  • the REPL removes excess characters at the end of the input line

    In Arc 3.1:

       arc> (readline) ;Fee fi fo fum
       " ;Fee fi fo fum"

    this is because Racket's reader reads up to the closing ")", leaving the rest of the input line in the input buffer, which is then read by readline.

    On the assumption that the REPL is being run from a terminal and thus there will always be a trailing newline (which sends the input line to the process), the ar REPL cleans out the input buffer up to and including the newline:

       arc> (readline) ;Fee fi fo fum
  • (coerce '() 'cons) now returns nil

    thus any list can be coerce'd to a "cons", even though the empty list isn't actually represented by a cons cell.

  • TCP ports no longer have associated custodians for force-close

    Arc 3.1's ac.scm says:

       ; there are two ways to close a TCP output port.
       ; (close o) waits for output to drain, then closes UNIX descriptor.
       ; mzscheme close-output-port doesn't work (just raises an error)
       ; if there is buffered output for a non-responsive socket.
       ; must use custodian-shutdown-all instead.

    I haven't been able to reproduce this behavior in any version of Racket or PLT Scheme that runs on my computer: in my testing sending data to a non-responsive client, close-output-port returns immediately and doesn't throw an error.

    Of course, if I'm wrong I'd be delighted to see an example demonstrating the problem.

  • embedding other runtimes based on ar

    Multiple runtimes can loaded and run within the same memory space. Each runtime has its own set of global variables, and can have a different set of definitions loaded. Thus the other runtimes can be a hacked version of ar, or have some other language than Arc loaded.

       arc> (use runtime)
       arc> (= a (runtime '(arc)))
       arc> a!+
       arc> (a!eval '(map odd '(1 2 3 4 5 6)))

    Note that it's easy to use functions defined in a different runtime, but this isn't a module solution that would let you use macros defined elsewhere.

The Arc Implementation Language (Ail)

Ail is a language intermediate between Racket and Arc, though closer to Racket than to Arc. The Arc runtime is written in Ail, and the Arc compiler compiles Arc to Ail.

The purpose of Ail is to make Arc more hackable, because it puts Arc's runtime implementation in Arc's namespace where it can be directly modified from Arc.

Ail is a terrible language for writing code in. It is like assembly language or bytecode: it's something you'd really rather have generated for you.

Ail can also be used to access Racket from Arc, though it's not a very convenient way to do that. However, a more friendly interface from Arc to Racket could probably be built that used Ail internally, or at least used a few calls to ail-code to get going.

Ail details:

  • Definitions and global variables are in Arc's namespace. Thus if you define a function foo in Ail, it becomes a function foo in Arc. Likewise, if code in Ail calls a function bar, and bar is defined in Arc, Arc's bar will be called.

  • Function calls such as (foo 1 2 3) are made using Racket's plain function call mechanism, and so can only call functions.

  • Racket identifiers are loaded into the namespace with a "racket-" prefix. Thus you can refer to Racket's + with racket-+, Racket's let with racket-let, and so on.

  • Ail code is not loaded in a Racket module, but is instead eval'ed one form at a time. This is like Arc's load or Racket's racket/load language.

    This means that Ail code isn't separated into compile-time and run-time phases like code in Racket's modules are; but it also means that we don't get some optimizations done for us that Racket's modules provide.

  • Racket macros can be used from Ail code. (But note this doesn't mean we can use Racket macros from Arc, because Arc's macro expansion and Racket's macro expansion are separate: all the Arc macros get expanded before Racket sees anything, and so there's no way to intermix Arc macros and Racket macros).

  • Ail code can be generated from Arc by using ail-code (and an Arc macro can expand into ail-code, and so you can write Arc macros which generate Ail code). For example, from Arc:

       (ail-code (racket-let ((foo 3))
                   (+ foo 2)))

    note that Arc's + is being called here, not Racket's. If we wanted Racket's +, we'd use racket-+.

    ail-code is only necessary when we need to use a Racket macro or special form, since Racket functions can be called directly from Arc. For example, from Arc:

       (racket-+ 3 4 5)

The Arc Compiler (ac)

In Arc 3.1, the Arc compiler operates on Racket lists: it takes Arc source code converted to a Racket list as input, and returns Racket code as a Racket list as output.

In ar, the Arc compiler operates on Arc lists: it takes Arc code in its original format as an Arc list, and returns a representation of Ail code as an Arc list.

This choice of representation means that extensions to the compiler can be written in Arc, and often using just plain Arc lists.

arc> (defrule ac (is s 'foo) '(prn "Fee-fi-fo-fum!"))
arc> foo

(note that the prn expression is in fact Ail code; it just happens to be written the same as it would be in Arc because it's a simple example).

In Arc nil represents both the symbol "nil" and the end of a list, but in Racket (and thus in Ail) the end of list (and thus the empty list as well) is represented by Racket's null: (). (In addition, an Arc list is constructed with mutable pairs and a Racket list is constructed with immutable pairs).

When converting an Arc list representing Ail code into a Racket list that can be fed to Racket's eval, we can choose some default for when we'd like nil to be translated into either a symbol or the end of list; but whatever choice we make there will be some other case that the default doesn't cover.

The choice made in ar (which seems to be the most useful the most often) is to translate an Arc nil into the symbol "nil" when it appears in the car of a pair, and to translate it into a Racket null "()" when it's in the cdr of a pair.

Thus in:

arc> (ail-code (racket-quote nil))

the input expression as an Arc list, showing the end of list terminator, looks like:

(racket-quote nil . nil)

the first nil is in the car of a pair, and the second nil is in the cdr. Thus the expression when converted to a Racket list looks like:

(racket-quote nil . ())

which is why Racket accepts the expression as a valid Racket list terminated with (), but the the final result output is "nil" instead of "()".

What this choice of default doesn't cover is representing an empty list in a car position. For example, suppose we wanted to specify the argument list for a Racket lambda:

`(racket-lambda ,args 123)

If args happens to be an empty list, we'd like the output to be the Ail equivalent of the Racket code:

(lambda () 123)

But that's not what happens. The input expression as an Arc list looks like:

(racket-lambda nil 123 . nil)

which gets converted to the Racket list:

(racket-lambda nil 123 . ())

for which the equivalent code in standard Racket would be:

(lambda nil 123)

i.e., what we end up with is a lambda that takes any number of arguments, that has a rest parameter called "nil".

We can tunnel arbitrary values through the Arc to Ail conversion using ar-tunnel:

`(racket-lambda ,(ar-tunnel (ar-list-fromarc args)) 123)

Now if args is the empty list nil, ar-list-fromarc will convert that to a Racket null (), and we'll end up with a Racket lambda which takes no arguments.


This project is derived from Paul Graham and Robert Morris's Arc 3.1 release; indeed, a goal is to incorporate as much of the original code with the fewest changes as possible.

Kartik Agaram discovered the queue bug (and provided a runnable example!), which was the original motivation for implementing Arc lists using Racket mpair's.

Waterhouse investigated the queue bug, determining that it is a garbage collection issue; this in turn gives us confidence that implementing Arc lists with Racket mpair's is in fact one way to fix the bug. (Note that waterhouse also provided a direct fix for Arc 3.1, so you don't need this runtime implementation just to get a fix for the queue bug).

Reflecting the Arc compiler into Arc was inspired by rntz's Arc compiler written in Arc.


  • explained why my definition of inline was broken by quote copying its value, and contributed the patch to make quote not do that.

  • provided the patch to make lexical variables take precedence over macros with the same name; waterhouse contributed the test.

  • provided a fix for the ssyntax precedence being different than Arc 3.1.


  • moved Arc's coerce and + functions out of ar; and made (coerce '() 'cons) return nil.

  • provided an implementation for defcall.

  • contributed testing by example.

  • made functions and macros print as #<fn> and #<mac>.

  • tests for extend and defrule.

  • discovered how to use Racket's make-derived-parameter to implement cwd in a non-kludgy way.

ret comes from Sean Kenney's kwizwiz.arc: