Enum decoders are cumbersome and not intuitive

Given a TS enum

enum Directions { North, South, East, West }

We currently write the decoder as

const directionsDecoder: Decoder<Directions> = oneOf(
  constant(Directions.North), 
  constant(Directions.South), 
  constant(Directions.East), 
  constant(Directions.West)
);

which isn't great.

It's worth noting that the most ideal syntax is not possible, since TS compiles out enums and hardcodes the enum values during compilation

const directionsDecoder = enumDecoder(Directions); // this doesn't work!

Something like this should be possible:

const enumDecoder = <E>(...enumValues: Array<E[keyof E]>): Decoder<E> =>
  oneOf(...enumValues.map((v) => constant(v)));

const directionsDecoder = enumDecoder(
  Directions.North, Directions.South, Directions.East, Directions.West
);

But no matter how I bash at the types I am unable to get the type checker to reconcile the relationship between the elements of the enum and the enum in total.

Read the existing oneOf and constant decoder APIs first, then investigate how TypeScript infers the relationship between enum members and the enum type. Done means an enumDecoder-style helper can accept the listed enum values, return a Decoder, and preserve type checking for the proposed usage.