···

       
62
       
62
       
 
       
```

     

       
63
       
63
       
 
       


     

       
64
       
64
       
 
       
But the package might need `pkgs.libbar_2` instead.

     

       
65
       
65
       
-
       
While the function could be changed to take `libbar_2` directly as an argument,

     

       
66
       
66
       
-
       
this would change the `.override` interface, breaking code like `.override { libbar = ...; }`.

     

       
67
       
67
       
-
       
So instead it is preferable to use the same generic parameter name `libbar`

     

       
68
       
68
       
-
       
and override its value in [`pkgs/top-level/all-packages.nix`](../top-level/all-packages.nix):

     

       
65
       
65
       
+
       
While the `libbar` argument could explicitly be overridden in `all-packages.nix` with `libbar_2`, this would hide important information about this package from its interface.

     

       
66
       
66
       
+
       
The fact that the package requires a certain version of `libbar` to work should not be hidden in a separate place.

     

       
67
       
67
       
+
       
It is preferable to use `libbar_2` as a argument name instead.

     

       
69
       
68
       
 
       


     

       
70
       
70
       
-
       
```nix

     

       
71
       
71
       
-
       
{

     

       
72
       
72
       
-
       
  libfoo = callPackage ../by-name/so/some-package/package.nix { libbar = libbar_2; };

     

       
73
       
73
       
-
       
}

     

       
74
       
74
       
-
       
```

     

       
69
       
69
       
+
       
This approach also has the benefit that, if the expectation of the package changes to require a different version of `libbar`, a downstream user with an override of this argument will receive an error.

     

       
70
       
70
       
+
       
This is comparable to a merge conflict in git: It's much better to be forced to explicitly address the conflict instead of silently keeping the override - which might lead to a different problem that is likely much harder to debug.

     

       
75
       
71
       
 
       


     

       
76
       
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## Manual migration guidelines

     

       
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73