Interestingly, the Lua programming language actually evolved from configuration files: https://www.lua.org/history.html (and is still officially deemed useful for writing them)
[1] An example: https://github.com/spc476/mod_blog/blob/master/journal/blog....
Lua is great.
John Ousterhout explained in one of his early TCL papers that, as a "Tool Command Language" like the shell but unlike Lisp, arguments were treated as quoted literals by default (presuming that to be the common case), so you don't have to put quotes around most strings, and you have to use punctuation like ${}[] to evaluate expressions.
TCL's syntax is optimized for calling functions with literal parameters to create and configure objects, like a declarative configuration file. And it's often used that way with Tk to create and configure a bunch of user interface widgets.
Oliver Steel has written some interesting stuff about "Instance-First Development" and how it applies to the XML/JavaScript based OpenLaszlo programming language, and other prototype based languages.
Instance-First Development: https://blog.osteele.com/2004/03/classes-and-prototypes/
>The equivalence between the two programs above supports a development strategy I call instance-first development. In instance-first development, one implements functionality for a single instance, and then refactors the instance into a class that supports multiple instances.
>[...] In defining the semantics of LZX class definitions, I found the following principle useful:
>Instance substitution principal: An instance of a class can be replaced by the definition of the instance, without changing the program semantics.
In OpenLaszlo, you can create trees of nested instances with XML tags, and when you define a class, its name becomes an XML tag you can use to create instances of that class.
That lets you create your own domain specific declarative XML languages for creating and configuring objects (using constraint expressions and XML data binding, which makes it very powerful).
The syntax for creating a bunch of objects is parallel to the syntax of declaring a class that creates the same objects.
So you can start by just creating a bunch of stuff in "instance space", then later on as you see the need, easily and incrementally convert only the parts of it you want to reuse and abstract into classes.
What is OpenLaszlo, and what's it good for? http://www.donhopkins.com/drupal/node/124
Constraints and Prototypes in Garnet and Laszlo: http://www.donhopkins.com/drupal/node/69
All configuration files were Tcl data structures that were sourced on server start.