Error handling

This page is a collection of facilities and tools from nix, nixpkgs and NixOS for error handling and debugging. You can use them to convey configuration errors to users or to debug nix expressions trough interactive or print debugging.

This article or section needs expansion.

Reason: This page is currently being worked on (04.05.2023) and desperately needs usage examples. (Discuss in Talk:Error handling#)
Please consult the pedia article metapage for guidelines on contributing.

In most cases you will want to stick to the highest level abstraction: config.warnings or lib.assertMsg

<give examples for both>

Nix

The nix language has a construct to help with printing messages.

• assert: throw an error (see Nix manual: Assertions)

The nix language also comes with some related builtin functions:

• throw: throw an error with a message
• abort: same as throw, but always stop evaluation
• trace: print to stderr
• traceVerbose: print, but only when in --trace-verbose mode
• break: breakpoint when in --debugger mode
• tryEval: catch throws and asserts

Commonly, assert is combined with throw to generate meaningful error messages: assert condition || throw "message";. This pattern is essentially how lib.assertMsg works (see Sec. nixpkgs).

nixpkgs

There are three main facilities for printing errors and do print debugging in nixpkgs:

• lib.trivial.* (see nixpkgs manual: lib.trivial)
  • lib.throwIf and throwIfNot
  • lib.warn, warnIf and warnIfNot
• lib.debug.*: tracing functions with some pretty printing
• lib.asserts.*: assert functions

These facilities also expose their attributes directly via lib.* (e.g. lib.throwIf).

Nixpkgs also has a debugging facility like nix's break: the breakpointHook.

NixOS

The NixOS module system again wraps these library functions and makes them available via module options (see NixOS manual: Assertions/Warnings):

• config.warnings = [];
• config.assertions = [];

An example for a debugging facility in NixOS is running NixOS tests interactively.

Debugging

As mentioned above, you can use break to debug nix code, breakpointHook to debug nix builds and interactive tools to debug NixOS tests.