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The original was posted on /r/nixos by /u/ylbeethoven on 2025-07-30 08:43:10+00:00.

When I thought I knew nix a little after using for a few years and suddenly it won’t build and the error log gave me almost nothing to debug.

➜ home-manager switch --flake /home/xxx/.dotfiles/nix/.#xxx@amd-nixos
error:
 … while evaluating a branch condition
 at /nix/store/a22am5r2smfwas9gh9gsfmfmcl65rzbg-source/lib/lists.nix:142:18:
 141| len = length list;
 142| fold' = n: if n == len then nul else op (elemAt list n) (fold' (n + 1));
 | ^
 143| in

… while calling the ‘length’ builtin at /nix/store/a22am5r2smfwas9gh9gsfmfmcl65rzbg-source/lib/lists.nix:141:13: 140| let 141| len = length list; | ^ 142| fold’ = n: if n == len then nul else op (elemAt list n) (fold’ (n + 1));

… while evaluating the option `assertions’:

… while evaluating definitions from `/nix/store/s4vibs4bafvw3m8cbhdyf417gxsvdxdr-source/modules/programs/firefox’:

(stack trace truncated; use ‘–show-trace’ to show the full, detailed trace)

error: The option programs.firefox.profiles' in /nix/store/s4vibs4bafvw3m8cbhdyf417gxsvdxdr-source/modules/programs/firefox’ is already declared in `/nix/store/d9hd7jqywksl91rsaqah9phc4vskcrpi-source/modules/home-manager/firefox.nix’.

the --show-trace is verbose and difficult to read as well.

P.S It turns out an issue from catppuccin/nix

I really hope nix can provide better error logs.