[iov-types] Enum tagging of basic types

[will-iov]

Proposal for handling subtypes using const enum for tagging. Note that specifying the type of a variable in addition to casting is probably overkill in most situations, just leaving it in here so you can see the compatibility (or incompatibility if you change the type of the variable from e.g. PrivateKeyString to PublicKeyString).

Disadvantages

• Using an empty enum is pretty counterintuitive, not very semantic. (Maybe another solution comes along via Tag types microsoft/TypeScript#4895 .)
• Compared to aliases (e.g. to a normal string), it requires lots of casting.

Advantages

• Besides the enum issue, actually very semantic: a PrivateKeyString is a PrivateKey and a string: we even get to see the relationship between e.g. private key buffers and strings.
• Doesn't pollute the source code outside of the typing system.

[ethanfrey]

This looks quite nice and you achieve plenty with the const enum and as.

My comment on github got a link to this response: microsoft/TypeScript#202 (comment) and it seems even more "idiomatic" to replace eg.

const enum PublicKey {}

with

declare const PublicKey: unique symbol

not sure the difference, but seems like some smart ts people were discussing this.

[will-iov]

Ooh, that could be a better approach, definitely more idiomatic. It would be nice if it was documented somewhere! The best I found on the typescript website was a changelog with a pretty minimal mention of unique symbol. I'll merge this for now and make a new PR switching to unique symbol for further discussion.

[ethanfrey]

I added that new pr already :)

But yeah, let's discuss approaches there, this was ready to merge and that change doesn't affect the public api