[Nix-dev] multiple versions of the same software? / collisions

[Roger Qiu]

That's a problem though, there shouldn't be much of a problem of having 
both versions of the package available in a single environment, as long 
as the second package is namespaced appropriately, in this case it's 
just a shell alias. This should be possible with Nix. There are 
situations where it makes sense to have multiple versions if say you're 
doing some CI work involving different versions of  a runtime.


On 4/08/2016 10:07 PM, Guillaume Maudoux (Layus) wrote:
> Hi,
>
> I think you are putting your efforts in the wrong direction.
> Nix is not designed to install both python 3.4 and 3.5 in the same
> environment.
>
> What it can do however is cache both in the store, so you can get an
> environment with python 3.4 with
> nix-shell -p python34
> and an environment with python 3.5 with
> nix-shell -p python35
>
> Nix can provide you with as many different environments at the same time.
> You can have a shell with python 3.4 running besides a shell with python
> 3.5.
> You should however not install both in your default environment (nix-env
> -i) as it won't work.
>
> May I refer you to https://nixos.org/wiki/Development_Environments ?
>
> Regards,
> -- Layus.
>
>
> Le 03/08/16 à 16:43, Roland Koebler a écrit :
>> Hi,
>>
>> I think I've now found out (at least partially), although this could
>> result in a package being installed multiple times.
>>
>>> It appears that the flag setting only works on already installed packages.
>> Yes, and if I use
>>
>>      $ nix-env -i python3-3.5.2
>>      ...
>>      $ nix-env --set-flag priority 10 python3-3.5.2
>>      ...
>>      $ nix-env -i python3-3.4.5 python3-3.5.2
>>      replacing old ‘python3-3.5.2’
>>      installing ‘python3-3.4.5’
>>      installing ‘python3-3.5.2’
>>
>> it does not work, since the last command first removes python3-3.5.2,
>> then probably the flag is lost, and then installs it again.
>>
>>> Also I'm not sure, but even if there are name collisions in the user
>>> environment, it should still be possible to install both packages but only
>>> use one of the names, and then create your name alias to the non-named
>>> package. However I do not know how this can be done.
>> I think I've found out. The following works:
>>
>>      $ nix-env -i python3-3.5.2
>>      $ nix-env --set-flag priority 10 python3-3.5.2
>>      $ nix-env -i -P python3-3.4.5
>>
>> --preserve-installed / -P was the relevant switch.
>> However, this can have some strange side-effects, since -P can
>> cause a package to be installed "multiple times":
>>
>>      $ nix-env -i -P python3-3.4.5
>>      ...
>>      $ nix-env -i -P python3-3.4.5
>>      ...
>>      $ nix-env -i -P python3-3.4.5
>>      ...
>>      $ nix-env -q
>>      ...
>>      python3-3.4.5
>>      python3-3.4.5
>>      python3-3.4.5
>>      python3-3.4.5
>>      ...
>>
>> This could then be "resolved" by:
>>
>>      $ nix-env -e python3-3.4.5
>>      uninstalling ‘python3-3.4.5’
>>      uninstalling ‘python3-3.4.5’
>>      uninstalling ‘python3-3.4.5’
>>      uninstalling ‘python3-3.4.5’
>>      $ nix-env -P -i python3-3.4.5
>>      installing ‘python3-3.4.5’
>>
>> But it still looks strange.
>> Is this intentional?
>>
>> Roland
>>
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