UCL: BEAM-friendly bindings for libUCL

LibUCL is a human-friendly, automation-oriented parser and generator for Universal Configuration Language or UCL. UCL's main benefits are:

• human readable and editable with comments
• machine parsable
• conversion to/from YAML, JSON
• macro and include-file support
• lightning-fast parser

UCL is a NIF-based binding to the reference implementation LibUCL, and currently requires that library to be pre-installed on your system.

It does not use dirty schedulers, nor does it time-slice NIF calls, as this would make the code significantly more complex for a function that is unlikely to be run frequently within the BEAM.

UCL comes with friendly functions for both Elixir and Erlang, we are all friends here.

Installation & Compilation

Dependencies

libucl is obviously a dependency, and the usual UNIX build chain is required.

You will need Erlang/OTP 25 or higher, because the build tool enc is a pre-compiled escript.

• FreeBSD: textproc/libucl devel/rebar3
• OSX: brew install libucl
• Debian: build-essential from base erlang-dev erlang-nox from erlang-solutions rebar3 from https://rebar3.org/
• OpenBSD: erlang-25 elixir from base rebar3 from https://rebar3.org/

On some systems, LibUCL is missing and you'll need to build it. On Debian, there is a conflicting compression library of the same name.

Building LibUCL from source

OpenBSD

$ doas pkg_add pkgconf autogen autoconf-archive autoconf-2.69 \
    automake-1.11 libtool
$ tar xzf libucl-*.gz
$ cd libucl*
$ ./autogen.sh
$ ./configure
$ make
$ doas make install

Debian

$ sudo apt install autoconf libcurl4-openssl-dev automake libtool \
    autoconf-archive pkg-config
$ tar xzf libucl-*.gz
$ cd libucl*
$ ./autogen.sh
$ ./configure --prefix=/usr
$ make
$ sudo make install

Installation

Via git clone:

$ export PATH=$PATH:/usr/local/lib/erlang25/bin:$HOME/.mix
$ mix local.rebar
$ rebar3 do clean, compile, ct

Via hex in the usual fashion:

• Elixir: add {:ucl, "~> 1.0"} to mix.exs
• Erlang: add {ucl, "1.1.0"} to rebar.config

Compilation should work on all UNIX-like OS out of the box. There is o Windows support planned, but if you can compile libucl on Windows, get in touch.

Tests are available as usual via rebar3 ct, and long-running property tests via rebar3 proper. The property tests are effectively fuzzing the NIF by injecting random binary() data and expecting it to return a classic {ok | error, any()} tuple, and take a considerable amount of time (several hours):

$ time rebar3 proper
... 3 hours later ...
00% {60000,66000}
10% {54000,60000}
09% {48000,54000}
10% {42000,48000}
10% {36000,42000}
09% {30000,36000}
09% {24000,30000}
09% {18000,24000}
09% {12000,18000}
10% {6000,12000}
9% {0,6000}
===> 1/1 properties passed
OK: Passed 500000 test(s).
11005.99 real     11039.08 user        86.36 sys

Although UCL is written in Erlang, and uses rebar3 ,it should compile cleanly as a dependency on any BEAM language.

Usage

iex> UCL.to_json(":foo")
     {:error, :ucl_invalid}
iex> "foo: true" |> UCL.to_json! |> Jason.decode!
     %{"foo" => true}

1> ucl:to_json(<<"foo: true">>).
    {ok,<<"{\n    \"foo\": true\n}">>}

The implementation is intended for functional transformation of config data held in memory, and thus a large portion of the LibUCL API has been summarily ignored. If you need something please let us know.

Universal Configuration Language Syntax

UCL is heavily infused by nginx configuration as the example of a convenient configuration system. However, UCL is fully compatible with JSON format and is able to parse json files. For example, you can write the same configuration in the following ways:

• in nginx like:

param = value;
section {
    param = value;
    param1 = value1;
    flag = true;
    number = 10k;
    time = 0.2s;
    string = "something";
    subsection {
        host = {
            host = "hostname";
            port = 900;
        }
        host = {
            host = "hostname";
            port = 901;
        }
    }
}

• or in JSON:

{
    "param": "value",
    "param1": "value1",
    "flag": true,
    "subsection": {
        "host": [
        {
            "host": "hostname",
            "port": 900
        },
        {
            "host": "hostname",
            "port": 901
        }
        ]
    }
}

Improvements to the json notation.

There are various things that make ucl configuration more convenient for editing than strict json:

General syntax sugar

• Braces are not necessary to enclose a top object: it is automatically treated as an object:

"key": "value"

is equal to:

{"key": "value"}

• There is no requirement of quotes for strings and keys, moreover, : may be replaced = or even be skipped for objects:

key = value;
section {
    key = value;
}

is equal to:

{
    "key": "value",
    "section": {
        "key": "value"
    }
}

• No commas mess: you can safely place a comma or semicolon for the last element in an array or an object:

{
    "key1": "value",
    "key2": "value",
}

Automatic arrays creation

• Non-unique keys in an object are allowed and are automatically converted to the arrays internally:

{
    "key": "value1",
    "key": "value2"
}

is converted to:

{
    "key": ["value1", "value2"]
}

Named keys hierarchy

UCL accepts named keys and organize them into objects hierarchy internally. Here is an example of this process:

section "blah" {
	key = value;
}
section foo {
	key = value;
}

is converted to the following object:

section {
	blah {
		key = value;
	}
	foo {
		key = value;
	}
}

Plain definitions may be more complex and contain more than a single level of nested objects:

section "blah" "foo" {
	key = value;
}

is presented as:

section {
	blah {
		foo {
			key = value;
		}
	}
}

Convenient numbers and booleans

• Numbers can have suffixes to specify standard multipliers:
  • [kKmMgG] - standard 10 base multipliers (so 1k is translated to 1000)
  • [kKmMgG]b - 2 power multipliers (so 1kb is translated to 1024)
  • [s|min|d|w|y] - time multipliers, all time values are translated to float number of seconds, for example 10min is translated to 600.0 and 10ms is translated to 0.01
• Hexadecimal integers can be used by 0x prefix, for example key = 0xff. However, floating point values can use decimal base only.
• Booleans can be specified as true or yes or on and false or no or off.
• It is still possible to treat numbers and booleans as strings by enclosing them in double quotes.

General improvements

Comments

UCL supports different style of comments:

• single line: #
• multiline: /* ... */

Multiline comments may be nested:

# Sample single line comment
/*
 some comment
 /* nested comment */
 end of comment
*/

Thanks

• Paul Davis, whose well documented NIF examples made writing this almost cake