[stevan]

> I could be misunderstanding you, but this does not sound functional to me as `spec` seems to have access to previous inputs to `spec` invocations. What am I missing here?

In a stateful system (where outputs depend on previous inputs) you need to have access to all previous inputs in order to determine the output of some input, right? Also keep in mind that this is a specification, not an efficient implementation. If you'd implement it like this you'd have to recompute the output based on the inputs over and over again for each new input. And yeah, you can think of it as some kind of functional composition as nine_k pointed out.

> This sounds very similar to DDD (like as described as the DDD book at the end of the blog post).

Yeah, I think DDD gets this right: some lightweight methods (e.g. event storming) around inputs and outputs. But yeah, Cleanroom did this in the 80s.

> At IBM direct, or are there any particular resources you can point to which you found useful?

On this particular topic, see Harlan Mills' "Stepwise Refinement and Verification in Box-Structured Systems" (1988): https://trace.tennessee.edu/utk_harlan/16/