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:
Atomy AST open:
MyIfThenElse = Node class:
children(.condition, .then, .else)
bytecode(g, mod) := do:
done = g new-label
nope = g new-label
mod compile(g, @condition)
g gif(nope)
mod compile(g, @then)
g goto(done)
nope set!
mod compile(g, @else)
done set!
macro(my-if(~cond) then: ~then; else: ~else):
MyIfThenElse new(node line, cond, then, else)
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.
At the heart of Atomy is pattern-matching. Method definition and invocation, every assignment, block argument, macroexpansion, and tons of other things use it.
Methods in Atomy can have numerous branches, and can be extended at any time
with additional branches. For example, the naïve fib
is defined with three
branches:
0 fib := 0
1 fib := 1
Fixnum fib := (- 2) fib + (- 1) fib
These three branches are fused at runtime into one method on Fixnum
, which
performs the pattern-matching to decide which branch to invoke. The branches
are inserted based on the preciseness of their patterns, so the order of their
definition does not matter, and they don't have to all be in the same place.
In this case, the 1
and 0
pattern-matches are both more precise than
Fixnum
, so they are tried first.
This idea of pattern "precision" is crucial. Any arbitrary pattern can be
compared to another pattern to determine which is more precise. This is even
more important when it comes to macroexpansion, which is based on
pattern-matching QuasiQuote
s with the syntax tree.
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 require
ing 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
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.
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.