opam switch create . fails when _opam exists

[avsm]

Just showing @RyanGibb how to setup an opam environment from scratch, and we observed that it's quite confusing when opam switch create . fails that it's not idempotent. If an _opam directory exists, then it fails but doesn't give a hint on how to continue. We had to rm -rf _opam (which doesn't seem safe to a beginner) and then opam switch create . again once the underlying issue was fixed.

It would be nice if opam switch create would give a direction on how to recover from the fact that an _opam directory exists already, perhaps with a deletion instruction or a -f to continue regardless.

[rjbou]

Can you precise the use case ? opam display a message when a local switch is already existing, and ask to clean up in case of failing initialisation:

$ pwd
/tmp/5073
$ opam switch create --empty .
$ opam switch create --empty .
[ERROR] There already is an installed switch named /tmp/5073

# remove switch from known switches
$ vim ~/.opam/config
$ opam switch create --empty .
[ERROR] There already is an installed switch named /tmp/5073

# failing switch initilisation
$ opam switch remove .
Switch /tmp/5073 and all its packages will be wiped. Are you sure? [Y/n] y
$ opam switch create . ocaml.4.10.0

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml" {= "4.10.0"}]

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
∗ installed base-bigarray.base
∗ installed base-threads.base
∗ installed base-unix.base
[ERROR] User interruption


<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
┌─ The following actions failed
│ ⬇ fetch ocaml-base-compiler 4.10.0
└─ 
┌─ The following changes have been performed (the rest was aborted)
│ ∗ install base-bigarray base
│ ∗ install base-threads  base
│ ∗ install base-unix     base
└─ 
Switch initialisation failed: clean up? ('n' will leave the switch partially installed) [Y/n] y
$ ls _opam
ls: cannot access '_opam': No such file or directory

[RyanGibb]

The use case was having created a switch on one branch, checking out a new branch, and trying to create a new switch.

E.g.

$ git checkout main
$ opam switch create .
$ git checkout v1.0
$ opam switch create.
[ERROR] There already is an installed switch named /dirpath

I've since realized opam switch remove . is the intended operation to remove the existing switch, instead of rf -f _opam. I think @avsm was suggesting appending a message like 'Use opam switch remove to remove it', interactively prompting the user whether to delete it, and/or a -f force flag to remove any existing switches when creating a new one.

[rjbou]

Ok, I see better.
I'm wondering, on the process, @RyanGibb why/how did you get to create a new switch while you already had one?
From what I understand, you wanted the local switch to follow the branch change (related to #4984), why not use opam install or opam reinstall?

[RyanGibb]

That sounds like a better solution than what we were doing! Apologies, I think this is just a case of me being unfamiliar with OCaml's tooling. Perhaps prompting the user to opam install/opam reinstall would be better a approach than what we suggested?

[mroch]

My use case probably only applies to Meta employees, but we have a related issue. I'd love to see a -f force option to ignore that the dir already exists.

We use Eden for our source control, which uses a virtual filesystem to download files on demand and track all local changes. Writing into _opam is very slow because of the overhead of tracking all the files, which is unnecessary since _opam will never be committed. Eden can mount directories from elsewhere on disk so writes to those folders are untracked.

But mount creates the directory, even though it's empty.

[sabine]

Here's a simple (but admittedly impulsive/careless) way how new users end up with a broken switch:

1. User starts running someone else's instructions that happen to create a switch
2. User realizes that they need to do a different step first, so they CTRL+C abort in the middle of installing the switch and proceed in the correct position of the instructions.
3. Now, they have a broken switch and there's no hint on how to fix that when trying to create the switch again.

Saving grace is that searching the Internet for [ERROR] There already is an installed switch named brings you right to this issue, where you find the solution.

For this use case, either of the proposals mentioned by @RyanGibb (interactively prompting, or appending a message that gives a hint on how to remove the switch) would be helpful, as it means that the new user doesn't need to do an Internet search to resolve their issue.

[rjbou]

What about prompting a link to a FAQ entry that summarises all cases (switch to remove and checkout new branch for the moment). The message will be cleaner and it is fast to update when a new case appears.

[sabine]

What about prompting a link to a FAQ entry that summarises all cases (switch to remove and checkout new branch for the moment). The message will be cleaner and it is fast to update when a new case appears.

That would be a big improvement for struggling new users, I think. The advantage of being able to update the FAQ without having to do a release is a strong point in favor of this solution.

That leaves @mroch's issue, which is separate from the new user experience.