3 ms·

See this is exactly the point where FP fans lose the rest of us.

• catgary 5y ago
  It’s a type constructor that extends to functions. It’s a functor.
• Twisol 5y ago
  You asked. I'm happy to dissect it to your level, but I don't know what you may be familiar with, and if you're not comfortable with type functions, I'm going to have a hard time motivating the extra bits a functor has. The List functor is a combination of the type function sending `T` to `List<T>` and the ... what do you want me to call it, a function function? implemented in Java as: static <T, S> Function<List<T>, List<S>> map(Function<T, S> f) { return (ts) -> { var ss = new ArrayList<S>(); for (final var t : ts) ss.add(f.apply(t)); return ss; }; } We call it as map(f).apply(ts). If we want to pass them both at once, we can rewrite the function so that it's not static, but it doesn't much change that it's a mapping operation. But it's not the `map` method that's the important part, so much as the idea that we can (or want to; can't in Java) pass `List` to any function that needs a Functor, which can then use whatever `map` implementation that functor has, without knowing what actual type function it was given. Really, I suspect it's neither the type constructor nor the map function that's the conceptual hurdle. It's abstracting over the type constructor; taking a type constructor as a parameter. You may have never used a language that supports that, or seen a need for it, and this may all seem like needless sophistry until you do.