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The problem is when you want to base the "value" part of any of these key/value pairs off some other value, you can't compute any new value - and you wind up duplicating variables in the configuration files (or worse, over several configuration files). This leads to someone inventing a new configuration-to-configuration converter to do what could be done in a macro. Key/Value pairs work in a rather limited portion of software, but most configuration formats are calling out for the ability to compute. Because they rarely work in practice, everyone forks the format to add their pet features, until some committee comes along and suggests "I know, I'll add all of your pet features into a universal format" - this thinking brought us to XML. Yaml, JSON and name-your-shitty-markup are continuations of this absurd line of thinking. When TS says Lisp, he doesn't necessarily mean "configure the world in common lisp", but he's talking about S-expressions - which are a 'universal' way of encoding trees as text (without the element/attribute ambiguity), which you can chose to either treat as data or as code. The in memory representation of parsed s-expressions is equivalent to their textual representation (homoiconicity), which means that you can write code to operate on these structures using only the knowledge of the text layout, and not some extra knowledge your programming language might use for encoding it (ie, objects). A configuration format using S-expressions need not be turing complete, as you can specify what should be data and what wants evaluating as code, if anything. You can place limits on what you want to be able to compute, by validating the input before evaluating it. As others have stated, the focus of configuration formats needs shifting from "syntactic flavor of the year" to proper validation of input. And the quickest path to validation of input is one where the parsing is automated - because Lisp does it for you. |