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error:
       … while calling the 'toJSON' builtin
         at /pwd/in.nix:1:1:
            1| builtins.toJSON { foo = a: 1; }
             | ^
            2|

       … while evaluating attribute 'foo'
         at /pwd/in.nix:1:19:
            1| builtins.toJSON { foo = a: 1; }
             |                   ^
            2|

       error: cannot convert a function to JSON
       at /pwd/in.nix:1:19:
            1| builtins.toJSON { foo = a: 1; }
             |                   ^
            2|