failure to upgrade "I couldn't find 1 constructor(s)"

[stew]

When I try to upgrade any of the dependencies of my metrics library I get the error:

metrics/oldmain> upgrade base base_2_9_1                        

  I couldn't complete the update because I couldn't find 1 constructor(s) for
  Synchronous.Syncrounous where I expected to. I found: []
  
  You can use `view Synchronous.Syncrounous` and
  `alias.term <hash> Synchronous.Syncrounous.<ConstructorName>` to give names to each constructor,
  and then try again.

[aryairani]

@stew What do you get when you view the type? (We can screenshare if you want)

@mitchellwrosen is currently working on some pre-condition checks for upgrade that will give more detail about what would go wrong and why and how to fix it.

Probably this can be fixed by creating the missing constructors.

[stew]

https://share.unison-lang.org/@stew/metrics/code/releases/1.0.0/latest/types/synchronous/Synchronous

[stew]

the constructor isn't missing

[mitchellwrosen]

@stew I tried cloning metrics just to reproduce; is there a branch on which I can try the command and see the error?

upgrade base base_2_9_1

[hojberg]

Could the subtle spelling mismatch Synchronous.Syncrounous be the cause?

[aryairani]

@stew Wait, so did we resolve this yesterday?

[pchiusano]

Summarizing the conclusion from this: the data type had an unnamed constructor. The upgrade process currently requires that constructors have names in order to upgrade a type, since it converts 1 same-named constructor to another.

I feel like an ideal experience in this situation would be:

• Do the upgrade, since it's a type with just one constructor, so there's no ambiguity about what should be mapped to what.
• Also do the upgrade if the hashes are the same in both cases.
• Warn the user "I upgraded Foo, but its constructor is missing a name. You can give it a name using <instructions>."

But I also feel like this is a somewhat rare case, and it will get more rare with the new move command that @tstat implemented.

I think the current message is reasonable but could maybe be worded a little more clearly and without weird notation. Maybe:

I couldn't complete the upgrade because one of your data types, Synchronous has unnamed constructors. upgrade relies on all types having named constructors to map constructors of each type to their upgraded versions.

You can fix this up via <instructions>
... then try the upgrade again.

[aryairani]

Agree about ideal experience, but with the current text-based system, we need an actual name to print, not just a mapping from #0 to #0. A message wording update is in #4503