The papercut thread - post your small annoyances/confusions here

Development
58

Another debugging papercut:

  • Stacktraces are basically useless. They’re in the inverse order of what you would expect (coming from most other programming languages) and this is not clearly indicated, and because they are not aligned in any way, it is difficult to scan for the thing you’re looking for. You’ll also often get illogical stacktraces due to lazy evaluation, and there seem to be no internal execution tracking features (that eg. remember the origin of a particular value) to compensate for this.

Edit: A concrete example I ran into yesterday… when using buildFHSUserEnv, it has an attribute named targetPkgs that expects a function which returns a list. When you accidentally pass in a list directly, you get this stacktrace:

error: while evaluating the attribute 'buildCommand' of the derivation 'sidequest-0.3.1' at /home/sven/projects/nixpkgs/pkgs/build-support/trivial-builders.nix:7:14:
while evaluating the attribute 'text' of the derivation 'sidequest-0.3.1-init' at /home/sven/projects/nixpkgs/pkgs/build-support/trivial-builders.nix:7:14:
while evaluating the attribute 'buildCommand' of the derivation 'sidequest-0.3.1-fhs' at /home/sven/projects/nixpkgs/pkgs/build-support/build-fhs-userenv/env.nix:190:3:
while evaluating the attribute 'passAsFile' of the derivation 'sidequest-0.3.1-usr-target' at /home/sven/projects/nixpkgs/pkgs/build-support/trivial-builders.nix:7:14:
attempt to call something which is not a function but a list, at /home/sven/projects/nixpkgs/pkgs/build-support/build-fhs-userenv/env.nix:31:17

… which does not mention targetPkgs anywhere, nor contains any stacktrace item that points at a line in the offending file, at all. It all just points at nixpkgs utility things that have no apparent relation to targetPkgs.

5 Likes