Explain generic types and add DebugLevel to Player

This simulation engine is intended for people who are interested in game design, not computer programming -- the engine wants to do all the engine stuff so the simulation can be implemented with less familiarity with the language. Generics, however, are not widely regarded as a "new programmer" thing -- even though they're surprisingly familiar, in the end  (slice-of-T, map-from-K-to-V). So a long comment explaining a bit about what's going on seems warranted.

cardsim/player.go

@@ -2,7 +2,55 @@ package cardsim
 
 import "math/rand"
 
-// Player stores all gameplay state for one player.
+// Player stores all gameplay state for one player at a specific point in time.
+// Game-specific data is stored in Stats.
+//
+// Player is a generic type -- see https://go.dev/blog/intro-generics for more
+// information on how these work. Think of "Player" as a "type of type" --
+// when you create one, you tell it what kind of data it needs to keep for
+// the simulation itself, and each Player that works with a different kind of
+// data is a different kind of Player and the compiler will help you with that.
+// This is the same idea as "slice of something" or "map from something to
+// something" -- different kinds of Players are different from each other and
+// "know" what type of data they use, so the compiler can tell you if you're
+// using the wrong type.
+//
+// Generic types have to use a placeholder to represent the type (or types --
+// consider maps, which have both keys and values) that will be more specific
+// when the type is actually used. They're called "type parameters", like
+// function parameters, because they're the same kind of idea. A function puts
+// its parameters into variables so you can write a function that works with
+// whatever data it gets; a generic type takes type parameters and represents
+// them with type placeholders so you can write a *type* that works with
+// whatever specific other types it gets.
+//
+// Just like function parameters have a type that says what kind of data the
+// function works with, type parameters have a "type constraint" that says what
+// kind of types the generic type works with. Go already has a familiar way
+// to express the idea of "what a type has to do": `interface`. In Go, type
+// constraints are just interfaces.
+//
+// But wait, why use generics at all? Can't we just use an interface in the
+// normal way instead of doing this thing? Well, yes, we could, but then the
+// compiler doesn't know that the "real types" for things matching these
+// interfaces all have to actually be the same type. The compiler will stop
+// you from putting an `Orange` into a `[]Apple`, but it wouldn't stop you from
+// putting a `Fruit` into a `[]Fruit` because, well, of course it wouldn't,
+// they're the same type.
+//
+// Different simulation games made with `cardsim` are different. Rules made for
+// simulating the economy of a kobold colony and mine wouldn't work at all with
+// data for a simulation about three flocks of otter-gryphons having a
+// territory conflict over a river full of fish. By using generics, the compiler
+// can recognize functions and data and types intended for different simulation
+// games and prevent you from using the wrong one, when it wouldn't be able to
+// if all this stuff was written for "some simulation game, don't care what".
+//
+// Generic interfaces (like `Card[C]`, `Rule[C]`, `InfoPanel[C]`, and more)
+// don't mean you have to write generics of your own. It's exactly the opposite!
+// Because the interface has this extra type in it, you only need to implement
+// the specific kind of interface that works with your game. There's more detail
+// on this in the comment on `Rule[C]`.
 type Player[C StatsCollection] struct {
 	Stats             C
 	Name              string
@@ -19,4 +67,6 @@ type Player[C StatsCollection] struct {
 	Turn              int
 	TemporaryMessages []Message
 	TemporaryPanels   []InfoPanel[C]
+
+	DebugLevel int
 }