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Coil: A Configuration Library
=============================
This directory contains a Dylan re-implementation of the Python COIL
configuration library. Although the code is completely new, it is
intended to be compatible with the Python library.
Differences from Python Implementation
======================================
- @package is not supported, for obvious reasons.
- The deprecated path syntax (i.e., a leading '=') is not supported.
- @map is not supported. This feature seems questionable to me due
to the complexity of the syntax, so I will wait until an actual
need develops before putting effort into it. --cgay
Introduction
============
Coil is a configuration file format that is parsed into a tree of
dict like Struct objects. The format supports inheritance, allowing
complicated configurations to be as compact as possible.
Design Goals
============
General design/implementation goals, some have been met, others are
still in progress.
- Support Twisted and non-Twisted reactor driven Python programs.
- Scalable to complex configurations, easily avoiding duplication.
- Orthogonal to code; code should not be required to know about the
config system used, it should be regular Python or Twisted code.
- Minimal boilerplate.
Text Format
===========
Coil provides the concept of a struct - an ordered list of key/value
pairs. Basic types are True, False, None, integers, floats, unicode
strings and lists of basic types. Here we define a single struct:
x: {
anInt: 1
aFloat: 2.4
aString: "hello"
andAList: [1 2 "hello" 4 None]
}
Whitespace doesn't matter, so these two are identical:
x: {a: 1}
x: {
a: 1
}
Structs can extends other structs: this means they inherit all
attributes from that struct. Extending is done with a special
attribute, @extends, with a value that is a path to another struct.
Paths can be relative, with a prefix of ".." meaning go up one level,
"..." go up two levels, etc., or absolute, starting from the special
location @root. In this example, y and z inherit from x and override
some of its attributes:
x: {a: 1 b: 2}
y: {
@extends: ..x # relative path
b: 3
}
z: {
@extends: @root.x # absolute path
b: 4
}
In this example y is the same as:
y: {a: 1 b: 3}
If the above example was a file called "foo.coil" and we did:
sub: {
@file: "foo.coil"
x: {c : 2}
}
then sub.z would be {c: 2 b: 4}.
For extending substructs there is a shorthand syntax. In this example
y and z both extend x, and have identical contents:
x: { a: {b: 1} }
y: {
@extends: ..x
a.b: 3
}
z: {
@extends: ..x
a: {
@extends: ..x.a
b: 3
}
}
Structs can also be used to import files, either given a path on the
filesystem, which can be absolute or relative to the current coil
file:
example: {@file: "/home/joe/test/example.coil"}
or give a path which is relative to the path of Python package which
is present in sys.path, for example the file "example.coil" which is
present in the coil.test package:
example: {@package: "ops.avs:./example.coil"}
Links can be used to have attributes whose value is determined based
on their context, i.e. at lookup time rather then at parse time.
Their syntax is like the paths used for @extends, except that they
have a "=" prefixed. For example, server1.myaddress.host will be the
same as server1.host in this example:
address: {host: =..host port: 1234}
server1: {
myaddress: {@extends: @root.address}
host: "www.example.com"
}
Finally, sub-structs can delete attributes provided by structs they
extend:
base: {x: 1 y: 2}
sub: {@extends: ..base
~x # sub now has no attribute "x"
}
References are also allowed within strings by using ${name}. For
example:
foo: "zomg"
bar: "${foo}bbq"
sub: { x: "foo is ${..foo}" y: "foo is ${@root.foo}" }
expands out to be:
foo: "zomg"
bar: "zomgbbq"
sub: { x: "foo is zomg" y: "foo is zomg" }
Emacs users may be interested in the coil mode provided by
misc/coil.el in the coil distribution tarball.
API Overview
============
The core of the Coil API is the Struct object. It is a dict-like
mapping object that knows its place in a tree and can reference items
anywhere in the tree.
Assume we have a file at /tmp/example.coil with the following
contents:
x: { y: {a: 2}
z: "hello"
list: [1 2 3]}
sub: {
@extends: ..x
y.b: 3
~z
}
We can then inspect the structure just like nested dict objects:
define module your-module
use coil;
...
end;
$ define variable conf = parse-coil-file("/tmp/example.coil");
$ conf["x"]["list"];
#[1, 2, 3]
$ conf["x"]["z"];
"hello"
$ conf.key-sequence;
#["x", "sub"]
$ key-exists?(conf, "sub"); // we deleted this with ~z
#f
$ conf["x"]["y"];
<struct {"a": 2}>
$ conf["sub"]["y"]; // inherited from x and added "b"
<struct {"a": 2, "b": 3}>
Also, we can access and items based on absolute and relative paths as
we can in the text format:
$ conf["x.z"];
"hello"
$ conf["@root.x.z"];
"hello"
$ define variable x = conf["x"];
$ x["..sub.y.b"];
3
$ conf["sub.y.c"] := 4;
$ conf["sub"]["y"]
<struct {"a": 2, "b": 3, "c": 4}>