[ndriscoll]

Think of `Numeric<T>` as analogous to `Comparator<T>`. T is the type you're doing stuff with, `Comparator<T>` tells sorting algorithms how to compare two Ts, and `Numeric<T>` tells mathematical algorithms how to add two T's, and what a "zero" value for T should be.

There are probably other reasons to do this that I'm forgetting, but off the top of my head,

1. You can implement `Numeric<T>` or `Comparator<T>` for types `T` that come from a library that you can't change (so can't make T implement an interface that the library author didn't implement), or where you don't want to introduce a dependency (you could have a type class `JsonDecoder[T]` that comes from a library, and you don't want the library with your T to introduce a dependency on the json library).

2. You can have more than 1 implementation for a given T. For JSON decoders/validators, you might provide a decoder which bails out on the first error, or one which tries to continue reading fields so it can return all errors (field X was a string, expected number. Field Y was expected to be >= 1024, etc.). For comparators, you might have a `.reversed` function to easily make a comparator that sorts in reverse order. etc.