lix/doc/manual/rl-next/better-errors-in-nix-repl.md

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---
synopsis: Concise error printing in `nix repl`
prs: 9928
Print top-level errors normally in `nix repl` Previously, errors while printing values in `nix repl` would be printed in `«error: ...»` brackets rather than displayed normally: ``` nix-repl> legacyPackages.aarch64-darwin.pythonPackages.APScheduler «error: Package ‘python-2.7.18.7’ in /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/cpython/2.7/default.nix:335 is marked as insecure, refusing to evaluate.» ``` Now, errors will be displayed normally if they're emitted at the top-level of an expression: ``` nix-repl> legacyPackages.aarch64-darwin.pythonPackages.APScheduler error: … in the condition of the assert statement at /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/lib/customisation.nix:268:17: 267| in commonAttrs // { 268| drvPath = assert condition; drv.drvPath; | ^ 269| outPath = assert condition; drv.outPath; … in the left operand of the OR (||) operator at /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/passthrufun.nix:28:45: 27| if lib.isDerivation value then 28| lib.extendDerivation (valid value || throw "${name} should use `buildPythonPackage` or `toPythonModule` if it is to be part of the Python packages set.") {} value | ^ 29| else (stack trace truncated; use '--show-trace' to show the full trace) error: Package ‘python-2.7.18.7’ in /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/cpython/2.7/default.nix:335 is marked as insecure, refusing to evaluate. ``` Errors emitted in nested structures (like e.g. when printing `nixpkgs`) will still be printed in brackets. Change-Id: I25aeddf08c017582718cb9772a677bf51b9fc2ad
2024-03-10 02:28:04 +00:00
cls: 811
---
Previously, if an element of a list or attribute set threw an error while
evaluating, `nix repl` would print the entire error (including source location
information) inline. This output was clumsy and difficult to parse:
```
nix-repl> { err = builtins.throw "uh oh!"; }
{ err = «error:
… while calling the 'throw' builtin
at «string»:1:9:
1| { err = builtins.throw "uh oh!"; }
| ^
error: uh oh!»; }
```
Now, only the error message is displayed, making the output much more readable.
```
nix-repl> { err = builtins.throw "uh oh!"; }
{ err = «error: uh oh!»; }
```
However, if the whole expression being evaluated throws an error, source
locations and (if applicable) a stack trace are printed, just like you'd expect:
```
nix-repl> builtins.throw "uh oh!"
error:
… while calling the 'throw' builtin
at «string»:1:1:
1| builtins.throw "uh oh!"
| ^
error: uh oh!
```