A real-life example. I worked on the call routing system that is used by large cable companies and a large satellite TV provider. When a call comes in, the co

A real-life example. I worked on the call routing system that is used by large cable companies and a large satellite TV provider. When a call comes in, the computers dynamically routes the call to the best place to handle the call. It needs to use all kinds of business rules to make the determination. For example, is there an outage in the customer's area? Are we running a special promotion in that area that this call is likely related to? Is the customer's account on hold with our credit department? All of these decision points, along with what could happen depending on how those decision points get evaluated, end up being things that are defined by the end user (that is, by the IT department at the call center, rather than by us, the original developers of this software). For example, if there's an outage in their area right now, play this specific outage message, then prompt them to see if they want to do something else-- a dynamic rule that'll only be in place for a few hours till the outage is done. Before you know it, you've essentially implemented a programming language within the programming language. This pattern ends up presenting itself again and again, in many systems that I've implemented. Other systems that I've done with user-defined rules that become essentially programming languages include fraud detection systems, call center flow guides/scripts, systems that help users customize their products for things like: engraving (for trophies or stationery), building computers, building ten-speed bikes, or configuring options for custom-built caskets. Regarding the customization of caskets or couches or any other custom-built good, this also becomes very much like a programming language. For example, if the user selects leather, then these fabric protection options become available and cost this much, but if they select fabric, then Scotch guard and some other options become available. If they select this group of options, then the manufacturing will have to be outsourced to such-and-such company because they're the only ones that can do that process, but otherwise we manufacture it over here. And so on. All of these things we want to be set up by the people who purchased the software from us, and are not to be coded directly by us when we write the software. This pattern of needing to express what amounts to code from within the application data becomes quite common once a system gets beyond a certain level of complexity.