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I believe this should work. 

```
pulumi.all(buckets.map(b =&gt; b.id.apply(id =&gt; `arn:aws:s3:::${id}`)))
```

The upside is that there is ultimately a lot of flexibility, but you are right that these cases can be subtle. We’re looking into a few ways to simplify many common cases further here. 

Yes I definitely think some helpers would be useful for typical case like JSON string formatting

For your answer, it will return a list of strings, but I want the full things to be JSON stringify in fact.
How can create a generic Pulumi.Output that launch a function of my own upon resolution

This `Pulumi.Output` are like promises to be resolved at execution

(I mean execution of the resource deployent)

Yes - `Output` is effectively a promise (it is actually a promise along with resource dependency information). 

Re: launch a function of your own - you can use `.apply(JSON.stringify)` off the previous answer to compute a `Output&lt;string&gt;`. 