Add string formatting function

[bluesign]

Issue to be solved

context: https://discord.com/channels/613813861610684416/1162086721471647874/1283181721562517607

It would be nice to have some basic string formatting.

Suggested Solution

No response

[SupunS]

Based on the discord discussion, this feature can be have two parts:

• Introduce a Stringer interface.
  • Make all string-convertible types are subtypes of this newly added interface
  • See the existing HashableStruct type for inspiration.
• Support string templates, where formatting can be inlined in the string-literal itself.
  • See Kotlin's string templates and Swift's string interpolation for some ideas.

[jribbink]

Just wanted to chime in here, I ran into a similar need tinkering on something a while back, but maybe with some more specific constraints.

I was looking to deterministically produce human-readable String representations of AnyStruct for the use-case of signing arbitrary Cadence values (something functionally equivalent to the output of log right now would be sufficient). Unfortunately, I couldn't really find a way to verify a signature for an arbitrary AnyStruct value on-chain right now.

The reason I'm commenting here is because depending on the implementation of the Stringer interface, it may or may not be suitable for this use-case. If the Stringer implementation is native to the language, then they could be safely used in this example. However, if developers were responsible for implementating the Stringer interface on structs, it would not be reliably safe, nor ergronomic for this use-case.

I am not saying that the Stringer interface has to be the solution to the problem I faced, however, I think it's important to point out the potential value of allowing developers to safely stringify structs which they may not control the implementation of.

(Not sure where this comment belongs, the linked FLIP seems to be limited to discussion around string interpolation)

[bluesign]

If the Stringer implementation is native to the language, then they could be safely used in this example

@jribbink I agree user defined Stringer is dangerous (I raised some concerns on this before), but also a quick note, in general for your use case, native Stringer can be also dangerous.

In general you need 3 properties; deterministic, uniqueness and immutable ( more like crypto hashing ) I think output of ‘log’ lacks some.

[SupunS]

Thanks @jribbink and @bluesign for the feedback! Given the concerns, we opened a FLIP for the Stringer interface specifically: onflow/flips#294 to discuss these concerns further and find a solution that works. We can also discuss in one of the upcoming working groups as well.