There's data definitions:

    data Constant           = Constant Double deriving (Show)

is a bit like `#[derive(Debug)] struct Constant(f64);`, ie. it's just a wrapper around a double and you can print it.

And there's typeclasses. Show is a typeclass. You can think of them as interfaces, so `instance Sqlite DB where open dsn = ...` is a bit like saying Sqlite is an implementation (instance) of the DB interface (typeclass). The typeclass itself could be defined like `class DB where open :: String -> IO ()` meaning the interface requires a function taking a string and having IO access (and of course you can require more than one function in your interface/typeclass).

The article also uses typeclasses with parameters. Parameters (and functions and variables) are written lowercase, while classes and constructors and such are capitalized, so

    class (Expr e) => Stringify e where
      stringify :: e -> String

    instance Stringify Constant where
      stringify (Constant x) = show x

means there's an interface Stringify that's parametrized over some type e (and that e in turn has to be in the Expr typeclass).

I tried to translate the article to Rust, but seems that the article's gotcha in Haskell is not necessarily an issue in Rust if we return `dyn OurTrait`, so now I'm even more confused. If anyone could take a look what I'm missing that would be welcome:

------------

Say, you want to write a simple language interpreter.

You have an `Expr` enum (sum type), with say, a `Constant(f64)` and a `BinaryPlus(Expression, Expression)`.

You can easily add a new function expecting an `Expr` indeed, but if you were to add a new variant to the `Expr` enum, you would have to go through the code and change every use site.

You can solve the issue by simply making a `struct Constant` and a `struct BinaryPlus`. Now you can just define a new trait for both of them, and you can use that `dyn trait` in your code -- you can add new functions and also new types without code change at use site!

So what's the issue?

In Haskell, a logic like

```

func example(runtime_val: f64) -> Expr {

  if (runtime_val is someRuntimeCheck())
    return Constant(..)

  else
    return BinaryPlus(.., ..)

}

```

can't compile in itself as `Expr` is a type class (=trait). Basically, in this mode Haskell awaits a concrete implementation (we actually get the exact same behavior with `impl Expr` in Rust), but here is my confusion: this can be circumvented in Rust with dyn traits..

Here is my (very ugly due to just hacking something together while pleasing the borrow checker) code showing it in Rust: https://play.rust-lang.org/?version=stable&mode=debug&editio...