Things I Don't Like in Configuration Languages

Spoiler: I created my own configuration language maml.dev

Here will be a list of all markup languages/configuration languages I found on the internet, and things I don't like about them.

YAML

- - 1
  - 2
- - 3
  - 4

YAML 1.2 is better than YAML 1.0. But still, the YAML specification is monstrous, and I don't get how people trying to implement it are not going insane. YAML contains too many features.

XML

<?xml version="1.0" encoding="UTF-8"?>
<catalog xmlns="http://ex.com/catalog"
         xmlns:cat="http://ex.com/catalog"
         xmlns:old="http://ex.com/oldcatalog">
  <item id="0042">
    <name>R&amp;D Handbook</name>
    <notes priority="1 2 3">n/a</notes>
  </item>
</catalog>

The era of XML is in the past. I remember the hype around XML, how it was thought to be a universal format, and people created a lot of stuff around it. But now it's dead.

JSON

{"name": "John", "age": 30, "car": null}

JSON is nice and JSON won. It's a universal, data-interchange format for the Web and applications. This is why I based my MAML on top of JSON. I fixed things that were a little bit annoying for me inside JSON.

TOML

[[servers.web]]
name = "frontend"
ip = "192.168.1.10"

[[servers]]
region = "eu-central"

[[servers.web]]
name = "backend"
ip = "192.168.1.11"

I don't get why the [table] is more readable, and I don't understand [[array of tables]] out of order. And the lack of null.

JSON5

{
  lineBreaks: 'Look, Mom! \
No \\n\'s!',
  hexadecimal: 0xdecaf,
  a: +.8675309, b: +8675309.,
}

Too many unnecessary features. Object key-value pairs are still unordered. No distinction between integers and floats.

HJSON

{
  Hjson: a string
  RegEx: \s+
  md:
    '''
    First line.
    Second line.
      This line is indented by two spaces.
    '''  
}

Unquoted strings and three different types of comments. Multi-line strings with significant indentation.

JSONH

{
    // use #, // or /**/ comments
    keys: without quotes,
    isn\'t: {
        that: cool? # yes
    }
}

JSONH is same as HJSON but different.

RJSON

{ 
  shopping-list: [ milk butter /* don't forget the beer */ beer ]
  a\ space : t\:em item\ with\ spaces
}

Strings without quotes, but only if they don't contain spaces or commas. No specification.

JSONC

{
    /*
      This is a multi-line comment
      that spans multiple lines
    */
    "name": "Jane Doe",
    "age": 25,
}

There are so many implementations and differences in implementations of JSON with comments. For example, this one has trailing commas. Still allows duplicate keys, still no integers, unordered key-value objects.

HCL

service "aws_ami" "ubuntu" {
  most_recent = true
  instance_type = var.instance_type != "" ? var.instance_type : "t3.micro"
}

I don't like nesting with service "http" "web_proxy" or ability to specify service multiple times in different parts of the file.

Pkl

class Bird {
  name: String
  hidden taxonomy: Taxonomy
}

pigeon: Bird = new {
  name = "Common wood pigeon"
  taxonomy {
    species = "Columba palumbus"
  }
}

This is not a markup language. This is a full-blown programming language. I honestly would just use TypeScript instead of this one.

RON

GameConfig( // optional struct name
    window_size: (800, 600),
    key_bindings: {
        "up": Up,
        "down": Down,
        "left": Left,
        "right": Right,
    },
)

Very Rust-centric.

EDN

#:demo{:id #uuid "de305d54-75b4-431b-adb2-eb6b9e546014"
 :ts #inst "2025-11-04T09:00:00Z"
 :nums [1 2 3 4/2 5.5M]
 :set #{:a :b :c}
 :rx #"[A-Z]+"
 :q #queue ["x" "y"]
 :tag #user{:name "Zed"}}

I guess Clojure developers love it. I'm not one of them.

HOCON

include required("foo.conf")
a : [ 1, 2 ] [ 3, 4 ]
data-center-east = ${data-center-generic} { name = "east" }
{ foo include : 42 }

Too many features. Is it some sort of programming language? And it looks like it's very focused on just one project.

NestedText

default repository: home
report style: tree
compact format: {repo}: {size:{fmt}}.  Last back up: {last_create:ddd, MMM DD}.
date format: D MMMM YYYY
size format: .2b

YAML variant with only strings. But I need booleans!

KDL

author "Alex Monad" email=alex@example.com active=#true
scripts {
  message """
    hello
    world
    """
}

KDL is much closer to XML with properties on nodes. But I like JSON-like data structures. They are easier to understand. Also, I don't like significant indentation on multi-line strings.

SDLang

// Trailing semicolons are optional
prop true;

anotherprod on; author "Peter Parker"

this-is.1_valid$Tag-Name

renderer:options "invisible"
physics:options "nocollide"

title \
    "Some title"

They do have true and false. Why adding on and off? By default, integers are 32 bits long. Suffix 10L is needed for 64-bits.

CUE

import "math"

piPlusOne: math.Pi + 1

"quoted field names": {
    "four^four":         math.Pow(4, 4)
}

I would just use TypeScript instead of this programming language. Too many features; the specification is too big.

Dhall

let makeUser = \(user : Text) ->
      let home       = "/home/${user}"
      let privateKey = "${home}/.ssh/id_ed25519"
      let publicKey  = "${privateKey}.pub"
      in  { home, privateKey, publicKey }
    {- Add another user to this list -}
in  [ makeUser "bill"
    , makeUser "jane"
    ]

Again, it is a programming language. So I guess only one implementation of Dhall exists?

Jsonnet

local my_function(x, y=10) = x + y;
{
  person1: {
    name: "Alice",
    welcome: "Hello " + self.name + "!",
  },
  person2: self.person1 { name: "Bob" },
  len: [
    std.length('hello'),
    std.length([1, 2, 3]),
  ]
}

Well, this is some sort of programming language with dynamic types. But there are so many good programming languages, so I don't know why this one needs to be used.

Nickel

let conf = {
  name = "NiCl",
  version  = "0.0.1$",
  description = "My cool app!"
} in

let SemanticVersion = fun label value =>
  let pattern = "^\\d{1,2}\\.\\d{1,2}(\\.\\d{1,2})?$" in
  if std.string.is_match pattern value then
    value
  else
    let msg = "invalid version number" in
    std.contract.blame_with_message msg label
  in

let AppSchema = {
  name | String,
  version | SemanticVersion,
  description | String,
} in

conf | AppSchema

Nice programming language. Not a markup language.

Starlark

def fizz_buzz(n):
    """Print Fizz Buzz numbers from 1 to n."""
    for i in range(1, n + 1):
        s = ""
        if i % 3 == 0:
            s += "Fizz"
        if i % 5 == 0:
            s += "Buzz"
        print(s if s else i)

fizz_buzz(20)

Starlark is a programming language. Good lack porting/reimplementing Starlark in another programming language.

UCG

let tuple = {
    inner = {
        field = "value",
    },
    list = [1, 2] + [3],
    "quoted field" = "quoted value",
};

This is just an implementation of a language. Not portable to other languages.

UCL

.include "${CURDIR}/path.conf"
.macro_name(param={key=value}) "something";
section "blah" "foo" {
    key = value;
}

Okay. An implementation of a configuration language. No specification. No other implementations.

Confetti

probe-device eth0 eth1

user * {
    login anonymous
    password "${ENV:ANONPASS}"
    machine 167.89.14.1
    proxy {
        try-ports 582 583 584
    }
}

Nice logo! But other than that, I found it difficult to understand what's going on in this example. What about escaping in strings?

HUML

website::
  ports:: 80, 443
  enabled: true
  factor: 3.14
  props:: mime_type: "text/html", encoding: "gzip"
  tags::
    - "markup"
    - "webpage"
    - "schema"

Nice idea to make YAML less worse. But it's still a YAML, with the same problems and significant indentation.

MAML

{
  project: "MAML"
  tags: [
    "minimal"
    "readable"
  ]

  # A simple nested object
  spec: {
    version: 1
    author: "Anton Medvedev"
  }

  # Array of objects
  examples: [
    {
      name: "JSON", born: 2001
    }
    {
      name: "MAML", born: 2025
    }
  ]

  notes: """
This is a multiline raw strings.
Keeps formatting as-is.
"""
}

So, I decided to create a specification for my own language. And I wanted a distinctive name, not JSON something. I wanted a nice name and a strict specification. All languages have trade-offs. MAML as well.

Things I Don't Like in Configuration Languages

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