Improving string2code conversion

[lidel]

https://github.com/ipfs/go-cid/pull/137/files is removing hardcoded mapping tables from go-cid,
and we need a replacement for people who will be migrating to go-multicodec

I want to document the best way way of doing string → code and code → string

My understanding is that:

Old way (go-cid)

cid "github.com/ipfs/go-cid"

// string → code
code := cid.Codecs["libp2p-key"]

// code → string
string := cid.CodecToStr[code]

New way (go-multicodec)

mc "github.com/multiformats/go-multicodec"

// string → code
var mc.Code code 
code.Set("libp2p-key")

// code → string
code.String()

@mvdan any thoughts if this is good enough?
Should we add StrToCodec and CodecToStr maps to go-multicodec, or is it not worth it?

[mvdan]

I'd prefer to not expose two ways of doing the same thing in the API. Is there a particular reason why you want the maps?

One big advantage with the method we chose is that it uses widespread interfaces (https://pkg.go.dev/fmt#Stringer and https://pkg.go.dev/flag#Value), so a multicodec.Code will work out of the box with fmt.Println and flag.Var. Another advantage is that methods are tied to the type, whereas global maps aren't - and they could be modified as well, since Go doesn't have read-only maps.

So, overall, I'd ask if there's a good reason that what we have right now isn't enough :) It's true that "string to code" is one more line now, but you could also draw lines in the opposite direction, such as fmt.Println(cid.CodecToStr[code]) now being just fmt.Println(code) thanks to the String method.

[lidel]

Thank you @mvdan!

In this context keeping it as-is makes sense, I will document it, no need to add anything new.