Error handling: Difference between revisions
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== Nix == | == Nix == | ||
The nix language has | The nix language has a construct to help with printing messages. | ||
* '''assert''': throw an error [ | * '''assert''': throw an error (see [https://nixos.org/manual/nix/stable/language/constructs.html?highlight=assert#assertions; Nix manual: Assertions]) | ||
The nix language | The nix language also comes with some related [https://nixos.org/manual/nix/stable/language/builtins.html; builtin functions]: | ||
* '''throw''': throw an error with a message | * '''throw''': throw an error with a message | ||
* '''abort''': same as throw, but always stop evaluation | * '''abort''': same as throw, but always stop evaluation | ||
* '''trace''': print to stderr | * '''trace''': print to stderr | ||
* '''traceVerbose''': print, but only when in --trace-verbose mode | * '''traceVerbose''': print, but only when in <code>--trace-verbose</code> mode | ||
* '''break''': breakpoint when in --debugger mode | * '''break''': breakpoint when in <code>--debugger</code> mode | ||
* '''tryEval''': catch throws and asserts | * '''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). [ | Commonly, assert is combined with throw to generate meaningful error messages: <code>assert condition || throw "message";</code>. This pattern is essentially how <code>lib.assertMsg</code> works (see Sec. nixpkgs). <ref>[https://github.com/NixOS/nixpkgs/issues/154292; throw vs assert discussion]</ref> | ||
== nixpkgs == | == nixpkgs == | ||
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There are three main facilities for printing errors and do print debugging in nixpkgs: | There are three main facilities for printing errors and do print debugging in nixpkgs: | ||
* lib.trivial.* [ | * lib.trivial.* (see [https://nixos.org/manual/nixpkgs/stable/#sec-functions-library-trivial; nixpkgs manual: lib.trivial]) | ||
** lib.'''throwIf''' and throwIfNot | |||
** lib.'''warn''', '''warnIf''' and warnIfNot | |||
* lib.debug.*: tracing functions with some pretty printing [ | * [https://nixos.org/manual/nixpkgs/stable/#sec-functions-library-debug; lib.'''debug'''.*]: tracing functions with some pretty printing <ref>[http://ryantm.github.io/nixpkgs/functions/library/debug/#sec-functions-library-debug; Nixpkgs/docs: lib.debug]</ref> | ||
* lib.asserts.*: assert functions | * [https://nixos.org/manual/nixpkgs/stable/#sec-functions-library-asserts; lib.'''asserts'''.*]: assert functions | ||
These facilities also expose their attributes directly via lib.* (e.g. lib.throwIf). | These facilities also expose their attributes directly via <code>lib.*</code> (e.g. <code>lib.throwIf</code>). | ||
Nixpkgs also has a debugging facility like nix's break: the breakpointHook | Nixpkgs also has a debugging facility like nix's <code>break</code>: the [https://nixos.org/manual/nixpkgs/stable/#breakpointhook; breakpointHook]. | ||
== NixOS == | == NixOS == | ||
The NixOS module system again wraps these library functions and makes them available via module options: [11 | The NixOS module system again wraps these library functions and makes them available via module options (see [https://nixos.org/manual/nixos/stable/index.html#sec-assertions-warnings; NixOS manual: Assertions/Warnings]): <ref>[https://github.com/NixOS/nixpkgs/blob/nixos-22.11/nixos/doc/manual/development/assertions.section.md; Nixpkgs/docs: Assertions]</ref> | ||
* config.warnings = []; | * <code>config.warnings = [];</code> | ||
* config.assertions = []; | * <code>config.assertions = [];</code> | ||
An example for a debugging facility in NixOS is running NixOS tests interactively | An example for a debugging facility in NixOS is running [https://nixos.org/manual/nixos/stable/index.html#sec-running-nixos-tests-interactively; NixOS tests interactively]. | ||
== Debugging == | == Debugging == | ||
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<!-- put debugging into a heading for search engine optimisation --> | <!-- put debugging into a heading for search engine optimisation --> | ||
As mentioned above, you can use break to debug nix code, breakpointHook to debug nix builds and | As mentioned above, you can use <code>break</code> to debug nix code, <code>breakpointHook</code> to debug nix builds and interactive tools to debug NixOS tests. | ||
== | == References == | ||