UCG has several formats it can convert a value into. Each one has various limits and restrictions on the values that can be converted to that format.

    JSON and YAML

    All UCG values can be converted into JSON or YAML. Integers and Floats are turned into Their equivalent number types. Tuples are turned into dicts. Lists are turned into arrays. Strings are turned into strings.

    The NULL or empty type is turned into null in JSON or YAML.


    TOML is very similar to the JSON and YAML formats. TOML has no notion of null though so NULL types result in a compile error when converting to TOML.

    Command Line Flags

    UCG is able to generate command line flags from ucg values following some translation rules for each type.

    • Empty values are not emitted.
    • Boolean values are translated to "true" and "false" repsectively.
    • Integers and Floats are rendered as numbers.
    • Tuples are rendered as --field value pairs for each field.
      • Nested tuples are ignored.
      • Nested Lists generate a new --field listitem pair for each item in the list.
      • For fields that are just one character in length use a single -. Use double dashes -- for fields that are longer than one character.


    let flags = {
        port = 8080,
        listen = "",
        verbose = NULL,
        dir = [
        "log.debug" = true,
        "log.format" = "json",

    Generates the following flags in a file with the .txt extension.

    -port 8080 --listen '' --verbose --dir 'some/dir' --dir 'some/other/dir' --log.debug true --log.format 'json'

    Environment Variables

    UCG is also able to generate environment variables from UCG values following a few translation rules.

    • Booleans, Integers, Floats, and Strings are output as is.
    • Lists are not output.
    • Tuples are output as a FIELD=value for each field with strings surrounded by single quotes.
      • Nested Tuples are not output.
    let vars = {
        USER = "me",
        HOSTNAME = "localhost",
        TWO = 2,
        VERBOSE = true,

    Generates the following in a file with a .env extension.


    Exec Script

    UCG has an exec converter that will generate a launch script in bash for applications that are configured via command line flags or environment variables. The exec converter uses a tuple with some required and optional fields to generate the script with.

    The command field is the path to the application the script will be launching. It is expected to be a string. There must only be one command field in the tuple.

    The args field is a list of command line arguments. The elements of the list can be strings or tuples. There must only be one args field in the tuple.

    The env field is a tuple representing the environment variables that will be set for the application. there must only be one env field in the tuple.


    let common_flags = {
        log-level = "debug",
        maxMem = "2048M",
    let script = {
        env = {
            API_KEY = "foo-key-and-stuff",
        command = "my-app",
        args = [
            "serve", "--port", "8080",
    out exec script;

    The script tuple above will generate the following bash script:

    #!/usr/bin/env bash
    # Turn on unofficial Bash-Strict-Mode
    set -euo pipefail
    exec my-app --log-level debug --maxMem 2048M serve --port 8080

    The items in the args should be either strings or tuples. The tuples are turned into flags using the builtin flag converter.


    XML can be output using a custom xml DSL.

    XML documents start with a tuple defining the document declaration and a root element.

        version = "1.1" // Optional, Defaults to 1.1
        encoding = "utf-8" // Optional, Defaults to UTF-8
        standalone = true // Optional Defaults to false
        root = { // Required defines the root element of the document.
            name = "top",

    Nodes in the document can be defined as either xml elements or text nodes. An xml element is defined as a tuple with a required name field and the three optional fields attrs, ns, and children.

    The name field is required and may optionally contain a namespace prefix like so: prefix:element-name.

    attrs if set is required to be a tuple or NULL. If NULL then no attributes are emitted If a tuple then each field is turned into an attribute on the element. The tuple should have only string values or null for each field. If NULL then the attribute is not set.

    ns if set is required to be either a string in which case the default xml namespace is set or a tuple with prefix and uri fields. which will define the prefix and uri for a namespace on that element.

    children is required to be a list of elements and text nodes or NULL. If NULL then no children are output.

        name = "ns:element-name",
        ns = {
            prefix = "myns",
            uri = "",
        attrs = {
            id = "foo",
        children = [
            // child elements go here.

    Text nodes can be output via two ways. Either just a string or as a tuple with a single field named text.

        text = "This is a valid text node",
    "This is also a valid text node";

    Example document

    let doc = {
        root = {
            ns = {
                prefix = "myns",
                uri = "",
            name = "top",
            attrs = {id = "foo"},
            children = [
                    name = "child1",
                    ns = "",
                    attrs = { attr1 = "value1", attr2 = "value2"},
                    children = [
                        "inner text node",
                            name = "myns:grandchild",
                            children = [
                                    text = "Another text node",
    out xml doc;

    This will result in the following xml document.

    <?xml version="1.0" encoding="UTF-8"?>
    <top xmlns:myns="" id="foo">
      <child1 xmlns="" attr1="value1" attr2="value2">inner text node<myns:grandchild>Another text node</myns:grandchild>


    We don't support character CDATA sections in our xml document DSL at this time.

    Next: HowTo Guides