[fiddlerwoaroof]

Yeah, there's something of a tension between the Perlis quote "It is better to have 100 functions operate on one data structure than 10 functions on 10 data structures" and Parse, don't validate.

The way I've thought about it, though, is that it's possible to design a program well either by encoding your important invariants in your types or in your functions (especially simple functions). In dynamically typed languages like Clojure, my experience is that there's a set of design practices that have a lot of the same effects as "Parse, Don't Validate" without statically enforced types. And, ultimately, it's a question of mindset which style you prefer.

[strawhatguy]

There's probably a case for both. Core logic might benefit from hard types deep in the bowels of unchanging engine.

The real world often changes though, and more often than not the code has to adapt, regardless of how elegant are systems are designed.

[fiddlerwoaroof]

Coalton ( https://coalton-lang.github.io ) is the sort of thing I like: a Haskell-style language hosted inside a very dynamic one with good interop.

[eddd-ddde]

Do those design practices protect you when you apply a refactor and now you don't know which call sites may be broken now?

[fiddlerwoaroof]

Yes