> Functional languages somewhat avoid shared or implicit state by making you pass everything in and then create something new on the way out.

Dataflow, in other words. You pass data through functions, and you know the whole thing works because the functions are composable because they aren't doing anything off to the side. No side-effects means you can think in a pipeline data flows through.

Spreadsheets achieve this quite nicely, and are the only successful visual programming language (pacem Microsoft's use of "Visual" as a marketing term for their decidedly text-based Visual Basic and Visual C++) in addition to being the forgotten programming paradigm, or at least the one most likely to be pointedly ignored.

Some of this might even have relevance for creating FPGAs based on dataflow models.