Commits on Mar 11, 2021

1. lib/modules: better error message if an attr-set of options is expected …

   I recently wrote some Nix code where I wrongly set a value to an option
   which wasn't an actual option, but an attr-set of options. The mistake I
   made can be demonstrated with an expression like this:
   
       {
         foo = { lib, pkgs, config, ... }: with lib; {
           options.foo.bar.baz = mkOption {
             type = types.str;
           };
           config.foo.bar = 23;
         };
       }
   
   While it wasn't too hard to find the cause of the mistake for me, it was
   necessary to have some practice in reading stack traces from the module
   system since the eval-error I got was not very helpful:
   
       error: --- TypeError --------------------------------------------------------- nix-build
       at: (323:25) in file: /nix/store/3nm31brdz95pj8gch5gms6xwqh0xx55c-source/lib/modules.nix
   
          322|         foldl' (acc: module:
          323|                 acc // (mapAttrs (n: v:
             |                         ^
          324|                                    (acc.${n} or []) ++ f module v
   
       value is an integer while a set was expected
       (use '--show-trace' to show detailed location information)
   
   I figured that such an error can be fairly confusing for someone who's
   new to NixOS, so I decided to catch this case in th `byName` function in
   `lib/modules.nix` by checking if the value to map through is an actual
   attr-set. If not, a different error will be thrown.

   

   Ma27 committed Mar 11, 2021
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