Mapped nullary tag is an inverse map upon the tag

[samhh]

A discovery worth documenting (https://github.com/unsplash/unsplash-web/pull/10389#discussion_r1280540272).

getCodecFromMappedNullaryTag builds a codec from:

• a -> b

• b -> m a

• [a]

• This is strikingly similar to inverseMap:

• a -> b

• b -> m a

• Enum a

The biggest difference is that in the former a is the member tag whereas in the latter it'd (typically) be the sum itself.

Finding the common ground between these could substantially reduce the boilerplate needed to write these mapped codecs.

[samhh]

I suppose it's getCodecFromPrimitiveMappedNullaryTag?