If I want to switch opam switch, should I use eval $(opam env --switch=<switch_name> --set-switch) or opam switch set <switch_name>? why are there two ways to do this?

Learning
1

I noticed that there were two ways to change switches. Why? What are the pros and cons and which one should one be using? which one is standard/best practice?

Is it true that this is the main difference?

The main difference between the two approaches is that opam switch set only affects opam, while eval $(opam env ...) affects the whole shell environment. So if you only need to use opam packages, you can use opam switch set . If you want to use the packages with other tools, you should use eval $(opam env ...) .

2

I don’t do either of those, I always do:

opam switch blabla
eval $(opam env)

For local development at least you do want to set the correct environment variables. Although there are some loopholes where even opam env doesn’t: `opam env` does not recalculate all paths correctly when local switch is moved · Issue #5423 · ocaml/opam · GitHub

3

I am a bit puzzled,

I have the following session:

oyer@ak-serveur2:~$ opam switch default
loyer@ak-serveur2:~$ type ocaml
ocaml est /home/loyer/.opam/default/bin/ocaml
loyer@ak-serveur2:~$ opam switch 5.0.0
loyer@ak-serveur2:~$ type ocaml
ocaml est /home/loyer/.opam/5.0.0/bin/ocaml

I don’t know how bash did know it has to upgrade its environment. The PATH has changed between the two switch command.

4

If I recall correctly when you install recent versions of opam it encourages you to let it install a switch hook so that it runs the eval $(opam env) automatically when you change switches. You can check your shell startup script to see if you have the hook installed.

5

Oh yes, the following code is execute at the start of bash:

PROMPT_COMMAND="_opam_env_hook;

6

Evil. I do very much hope this always stays optional. I’m already staying away from Nix because of this; I don’t want Ocaml to be next.

7

related but how to do change switch from within python: Is eval $(opam env --switch={switch} --set-switch) equivalent to opam switch set switch? and What is the difference between eval $(opam env --switch={switch} --set-switch) and opam switch set SWITCH? - Stack Overflow