神文：Elm 思維訓練 — Types as Sets

https://guide.elm-lang.org/appendix/types_as_sets.html

One of the most important techniques in Elm programming is to make the possible values in code exactly match the valid values in real life. This leaves no room for invalid data, and this is why I always encourage folks to focus on custom types and data structures.

In pursuit of this goal, I have found it helpful to understand the relationship between types and sets. It sounds like a stretch, but it really helps develop your mindset!

So the whole point here is that ruling out invalid data makes your code shorter, simpler, and more reliable. By making sure the set of possible values in code exactly matches the set of valid values in real life, many problems just go away. This is a sharp knife!

As your program changes, the set of possible values in code may start to diverge from the set of valid values in real life. I highly recommend revisiting your types periodically to make them match again. This is like noticing your knife has become dull and sharpening it with a whetstone. This kind of maintenance is a core part of programming in Elm.