Atomy

A DSL-oriented programming language targeting the Rubinius VM.

IRC: #atomo on freenode

Atomy provides the foundation for a language that grows with its users.

Many languages end up being snapshots of their initial design goals. The designers bake features right into the language's core, rather than implementing them as libraries. Eventually, this language may start to feel stale, and either mediocre or backwards-incompatible changes are introduced to breathe new life into it. This often occurs at the syntax level.

Not only does this lead to long transition periods, but it fragments the language and your projects. The move towards a backwards-incompatible version can be painfully slow. Hell, Ubuntu 12.04 still ships with Ruby 1.8, years after 1.9 came around.

Atomy avoids this by saying very little about language semantics at its core, instead providing a system that you can use to build the language you want. The core components are detailed below.

Reaching Atomy's goals requires a stable, deceptively simple grammar. It says nothing about language semantics, instead defining primitive "building blocks" which, when composed together, form the notation of the language.

The various forms are as follows:

• Word: a, foo-bar
• Constant: Foo, FooBar123
• Primitive: 1, 200
• Literal: 2.0, "foo"
• List: [1, 2, 3]
• Block: { a, b }, : a, b. The second notation is whitespace-aware.
• Infix: 1 + 1
• Postfix: no!
• Prefix: @foo
• Call: foo(bar, baz)
• Compose: 1 inspect
• Quote: 'foo
• QuasiQuote: `foo
• Unquote: ~foo

Note that these say nothing about variables, messages, methods, or functions. Blocks also don't have arguments; the notation for this is actually built up by composing simpler forms together:

[a, b]: a + b

This parses as a List composed with a Block. The AST looks like this:

Compose
  List
    Word (a)
    Word (b)
  Block
    Infix
      Word (a)
      Word (b)

On its own, this means nothing. If you try to compile this, you'll get an error - things like Compose and Call have no meaning on their own. This is where macros come in.

If Atomy's AST is a bunch of Lego bricks pieced together, the macro system is the imagination that gives it meaning.

Macros are defined using patterns that match arbitrary expressions. This is in contrast to most macro systems, which either use named macros (i.e. Lisps) or raw text substitution (C). Atomy's macros are nameless, and match on the AST itself, rather than source code.

For example, when a List is composed with a Block, we get a Block with arguments:

macro([~*args]: ~*body):
  Block new(node line, body, args)

Here we're using splice unquotes (~*foo) to match the contents of the list and block, and creating a new block with the original's contents and the given arguments. This macro is defined in the "core" Atomy library.

Note that we're creating the Block manually; macros can return any object as long as it knows how to compile itself. After expansion, nodes are sent bytecode(g, mod), where g is the code-generator and mod is the module being compiled. Users are free to define arbitrary nodes that do whatever they need to at the bytecode level. This is how things like if-then-else are implemented without being a primitive:

my-if-then-else = class:
  def(initialize(@if, @then, @else)) {}

  def(bytecode(gen, mod)):
    else = gen new-label
    done = gen new-label

    mod compile(gen, @if)
    gen goto-if-false(else)

    mod compile(gen, @then)
    gen goto(done)

    else set!
    mod compile(gen, @else)

    done set!

macro(my-if(~x) then: ~*y; else: ~*z):
  my-if-then-else new(x, `(do: ~*y), `(do: ~*z))

my-if(true)
  then: puts("1")
  else: puts("0")

For more information on the Rubinius VM bytecode, see the instruction set.

Normally though you probably won't be digging into bytecode to write macros. For most cases you'll probably just use Lisp-style quasiquotation:

macro(~x for(~*args) in(~c) when(~t)):
  names [tmp]:
    `(do:
        ~tmp = []
        ~c collect [~*args]:
          when(~t):
            ~tmp << ~x

        ~tmp)

(v * 3) for(v) in(0 .. 10) when(v even?)

Here we're implementing Python-style list comprehensions. We use names to generate temporary variable names to avoid collision.

Everything in Atomy is a closure. This differs from Ruby, where methods and class/module bodies do not capture local variables.

There are many places this comes in useful, but in terms of other Atomy features, it's good for assigning modules to variables at the top of a file, and defining helper functions for use in your exposed methods (see the following section for more info there).

Vaguely similar to CommonJS-style modules, require will result in a module object, rather than evaluating the file in some global scope.

Methods defined at the toplevel are defined on the file's module, and can be called by anyone requireing the file.

Macros defined in a file are local to its module. For another module to use them, they must call use rather than require. use will also bring the module's methods into the user.

For example, if we have a file a.ay:

use("atomy")
macro(bar): 42
def(foo(a)): a + bar

We can do this to invoke the foo method:

require("a") foo(2) -- => 44

Or we can use it to bring in its macros and methods:

use("a")
bar    -- => 42
foo(2) -- => 44

Methods are always public, and should be used only for things you want exposed as your API.

Functions replace private/helper methods. They are simply locals bound to a block that gets called with self as the self of its caller. They can do everything a method can, except they are not bound to a class or a module. To define a function, use fn(x): ... instead of def(x): ....

This may seem a little odd coming from Ruby, but in practice it simplifies your decision process when writing code. When defining something, it comes down to one question: should this be part of my public API? If not, define it as a function. It doesn't matter where you put it, as long as it's in scope, and there's no concern of others using it in production.