[chriswarbo]

> A "configuration language" by definition (in my opinion) should not be Turing complete.

One of the key insights of LISP is that s-expressions are a simple, universal format. Yes, they can be used for code, but they can also be used for static configuration data. In fact, LISP originally used s-expressions solely for data; code was meant to be written with m-expressions ( http://en.wikipedia.org/wiki/M-expression ). Once `eval` was implemented, s-expressions could be used for code and data, so the idea of m-expressions was abandoned.

> A configuration language is for storing state - key/value pairs or simple structures of primitive types, nothing more (or as little more as necessary), nothing less.

The trouble with "universal" formats like this is that there's no universal agreement on what's a "primitive type" (what happens when I write `0.1`? Are booleans primitive, or should we use `0` and `1`?) and what's a "simple structure" (can I make a circular list?). That in itself wouldn't be too bad, but these languages tend to hard-code special syntax to particular types and structures, so any types or structures we may want to add must either be second-class citizens, or would require hacking the parser.

[krapp]

I suppose you would have to draw a in the sand somewhere as far as types go, and it depends on the language. I know TOML allows dates as a type, which seems useful, but then you could just stick to ints and use Unix timestamps, I guess it depends on how strict you want to be.

But to me, a primitive type at least can't evaluate to anything other than itself, and also doesn't include pointers or references. In Lisp terms I guess it would be an atom? Likely at least numeric values, booleans (in whatever form), chars and some kind of tainted string (tainted in that it can't be evaluated as code, even if you try to do so.)

>and what's a "simple structure" (can I make a circular list?)

A simple structure as I define it is a collection that contains primitive types, so not, like, an array of function pointers or anything. Array, struct, tuple, map, etc. I think a circular list as a type might be interesting and definitely useful (does it exist anywhere?) If I ever design a programming language that's one of the things I want to add.