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

[Roland Koebler]

Hi,

I'm using Nix on Debian 8, and tried to install both Python 3.4 and 3.5
via Nix, which did not work, since one always replaced the other. So:

How does Nix determine if one package should replace an other?
And how could I prevent this?

Example:
  # Trying to install both Python 3.4 and 3.5 
  # (since I would like both versions for testing).

  $ nix-env -i python3-3.4.5
  installing ‘python3-3.4.5’
  ...

  $ nix-env -i python3-3.5.2
  replacing old ‘python3-3.4.5’
  installing ‘python3-3.5.2’

  So, this did not work. When trying to install both at once, it also fails:

  $ 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’
  building path(s) ‘/nix/store/r1ickgl36b4374zl8nlqfc25qnlr4lga-user-environment’
  Wide character in die at /nix/store/64jc9gd2rkbgdb4yjx3nrgc91bpjj5ky-buildenv.pl line 79.
  collision between ‘/nix/store/zg1f3g03v8i1pnr1hvm5mj285jwzr1ri-python3-3.4.5/bin/2to3’ and ‘/nix/store/06af6d830iawf
  builder for ‘/nix/store/sz3pbwznp8q59ggbqm15wilsa8n9jnph-user-environment.drv’ failed with exit code 255
  error: build of ‘/nix/store/sz3pbwznp8q59ggbqm15wilsa8n9jnph-user-environment.drv’ failed

  The given hint also doesn't work:

  $ nix-env --set-flag priority 10 python3-3.5.2
  setting flag on ‘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’
  ...
  (same output as above)


So, how could I install both?
(It's clear that python3 could only point to one, but I'm happy with 
using them as python3.4 and python3.5.)


One additional question: When I install an already installed package,
no action should be necessary. But the following happens. Why?

  $ nix-env -i python3-3.5.2
  replacing old ‘python3-3.5.2’
  installing ‘python3-3.5.2’


thanks
Roland