gcl 0.3.0

gcl -- Generic Configuration Language
=====================================

GCL is an abstract configuration language that can be dropped into any Python
project. It supports dictionaries with name-value pairs, all the basic types
you'd expect, lists, includes, and methods for abstraction.

Why not use JSON?
-----------------

JSON is good for writing complex data structures in a human-readable way, but
it breaks down when your config starts to become more complex. In particular,
JSON lacks the following:

* No comments, making it hard to describe what's going on.
* No expressions, so there are no ways to have values depend on each other
(e.g., `instances_to_start = expected_tps / 1000`.
* No abstraction, which makes it impossible to factor out common pieces of
config.
* All the double quotes I have to type make my pinkies sore! :(

Basic syntax
------------

GCL is built around named tuples, written with curly braces:

{
# This is a comment
number = 1;
string = 'value'; # Strings can be doubly-quoted as well
bool = true; # Note: lowercase
expression = number * 2;
list = [ 1, 2, 3 ];
}

The top-level of a file will be parsed as a tuple automatically, so you don't
write the braces there. Semicolons are considered separators. They may be
ommitted after the last statement if that aids readability.

The basic types you'd expect are supported: strings, ints, floats, bools and
mapped onto their Python equivalents. Lists are supported, but can't really be
manipulated in GCL right now.

Expressions
-----------

1 + 1
'foo' + 'bar'
80 * '-'

GCL has an expression language, looking much like other languages you're used
to. The evaluation model is mostly borrowed from Python, so things you expect
from Python (such as being able to use `+` for both addition and string
concatenation).

inc(1)

Function application also looks the same as in Python. There's currently no way
to define functions in GCL, but you can invoke functions passed in from the
external environment.

inc 1

If a function only has one argument, you can omit the parentheses and simply
put a space between the function and the argument.

tuple = {
foo = 3;
}

that_foo = tuple.foo;

Periods are used to dereference tuples.

http = include 'library/http.gcl';
server = http.Server {
port = 8080;
}

External files can be included with the built-in `include()` function. The
result of that expression is the result of parsing that file (which will be
parsed as a tuple using the default environment).


Tuple composition
-----------------

As a special case, a tuple can be applied to another tuple, yielding a new
tuple thats the merge of both (with the right tuple overwriting existing keys
in the left one).

This looks especially convenient when A is a reference and B is a tuple
literal, and you use the paren-less function invocation:

FooApp = {
program = 'foo';
cwd = '/tmp';
}

my_foo = FooApp {
cwd = '/home';
}

`my_foo` is now a tuple with 2 fields, `program = 'foo'` (unchanged) and
`cwd = '/home'` (overwritten).

This makes it possible to do abstraction: just define tuples with the common
components and inherit specializations from them.

Because tuple elements are lazily evaluated (i.e., only when requested), you
can also use this for parameterization. Declare keys without giving them a
value, to signal that inheriting tuples should fill these values:

greet = {
greeting;
message = greeting + ' world';
};

hello_world = greet { greeting = 'hello' }

If `message` is evaluated, but `greeting` happens to not be filled in, an
error will be thrown. To force eager evaluation (to try and catch typos), use
`eager()` on a tuple.


Requirements
------------

* Uses `pyparsing`.