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---
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synopsis: "Trace which part of a `foo.bar.baz` expression errors"
cls: 1505, 1506
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credits: Qyriad
category: Improvements
---
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Previously, if an attribute path selection expression like `linux_4_9.meta.description` it wouldn't show you which one of those parts in the attribute path, or even that that line of code is what caused evaluation of the failing expression.
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The previous error looks like this:
```
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pkgs.linuxKernel.kernels.linux_4_9.meta.description
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error:
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… while evaluating the attribute 'linuxKernel.kernels.linux_4_9.meta.description'
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at /nix/store/dk2rpyb6ndvfbf19bkb2plcz5y3k8i5v-source/pkgs/top-level/linux-kernels.nix:278:5:
277| } // lib.optionalAttrs config.allowAliases {
278| linux_4_9 = throw "linux 4.9 was removed because it will reach its end of life within 22.11";
| ^
279| linux_4_14 = throw "linux 4.14 was removed because it will reach its end of life within 23.11";
… while calling the 'throw' builtin
at /nix/store/dk2rpyb6ndvfbf19bkb2plcz5y3k8i5v-source/pkgs/top-level/linux-kernels.nix:278:17:
277| } // lib.optionalAttrs config.allowAliases {
278| linux_4_9 = throw "linux 4.9 was removed because it will reach its end of life within 22.11";
| ^
279| linux_4_14 = throw "linux 4.14 was removed because it will reach its end of life within 23.11";
error: linux 4.9 was removed because it will reach its end of life within 22.11
```
Now, the error will look like this:
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```
pkgs.linuxKernel.kernels.linux_4_9.meta.description
error:
… while evaluating the attribute 'linuxKernel.kernels.linux_4_9.meta.description'
at /nix/store/dk2rpyb6ndvfbf19bkb2plcz5y3k8i5v-source/pkgs/top-level/linux-kernels.nix:278:5:
277| } // lib.optionalAttrs config.allowAliases {
278| linux_4_9 = throw "linux 4.9 was removed because it will reach its end of life within 22.11";
| ^
279| linux_4_14 = throw "linux 4.14 was removed because it will reach its end of life within 23.11";
… while evaluating 'pkgs.linuxKernel.kernels.linux_4_9' to select 'meta' on it
at «string»:1:1:
1| pkgs.linuxKernel.kernels.linux_4_9.meta.description
| ^
… caused by explicit throw
at /nix/store/dk2rpyb6ndvfbf19bkb2plcz5y3k8i5v-source/pkgs/top-level/linux-kernels.nix:278:17:
277| } // lib.optionalAttrs config.allowAliases {
278| linux_4_9 = throw "linux 4.9 was removed because it will reach its end of life within 22.11";
| ^
279| linux_4_14 = throw "linux 4.14 was removed because it will reach its end of life within 23.11";
error: linux 4.9 was removed because it will reach its end of life within 22.11
```
Not only does the line of code that referenced the failing attribute show up in the trace, it also tells you that it was specifically the `linux_4_9` part that failed.
This includes if the failing part is a top-level binding:
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```
let
inherit (pkgs.linuxKernel.kernels) linux_4_9;
in linux_4_9.meta.description
error:
… while evaluating 'linux_4_9' to select 'meta.description' on it
at «string»:3:4:
2| inherit (pkgs.linuxKernel.kernels) linux_4_9;
3| in linux_4_9.meta.description
| ^
… while evaluating the attribute 'linux_4_9'
at /nix/store/dk2rpyb6ndvfbf19bkb2plcz5y3k8i5v-source/pkgs/top-level/linux-kernels.nix:278:5:
277| } // lib.optionalAttrs config.allowAliases {
278| linux_4_9 = throw "linux 4.9 was removed because it will reach its end of life within 22.11";
| ^
279| linux_4_14 = throw "linux 4.14 was removed because it will reach its end of life within 23.11";
… caused by explicit throw
at /nix/store/dk2rpyb6ndvfbf19bkb2plcz5y3k8i5v-source/pkgs/top-level/linux-kernels.nix:278:17:
277| } // lib.optionalAttrs config.allowAliases {
278| linux_4_9 = throw "linux 4.9 was removed because it will reach its end of life within 22.11";
| ^
279| linux_4_14 = throw "linux 4.14 was removed because it will reach its end of life within 23.11";
error: linux 4.9 was removed because it will reach its end of life within 22.11
```