Add String.to_existing_atom/2 to validate against a list of allowed atoms

[sabiwara]

• the second commit inlines it in the compiler in the case of a static list, using much faster pattern-matching (bench)
• the third commit special cases it in the type sytem so that we can narrow down the return type to :foo | :bar

I'd like to get proper type warnings if the variable is not a list of atoms, but not sure what the best way to do this.

[josevalim]

You can probably change remote_apply instead. do_remote customizes how the whole function is type checked. remote_apply changes only how you compute the return type. This means that, if someone passes to_existing_atom(string, "not a list"), we will type check it already. You don't have anything else to do. And the implementation of remote_apply is likely a simple call to list_hd!

[sabiwara]

I managed to make it work in 4456ce4.

And the implementation of remote_apply is likely a simple call to list_hd!

It seems we still need to run remote_apply/3 first to check the domain in our custom remote_apply/5 clause, otherwise it doesn't properly validate args against the signature? But it was fairly straightforward with this method.

[josevalim]

Do we want to check it is a non-empty list? As an empty list will always fail anyway?

[sabiwara]

Indeed, done in c3094ca

[josevalim]

Ship it! We likely want to add List.to_existing_atom too? It can be either here or in a separate PR :)

[sabiwara]

Ship it! We likely want to add List.to_existing_atom too? It can be either here or in a separate PR :)

Yes, I thought I'd just do it as a follow-up once this one is done 👍