Error handling: Difference between revisions
imported>Pogobanane m fix layouting |
imported>Carschandler m The previous example for config.warnings was returning a list inside of a list, which doesn't work properly |
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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 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. | ||
In most cases you will want to stick to the highest level abstraction: <code>config.warnings</code> or <code>lib.warn</code> and its relatives. | |||
<syntaxHighlight lang=nix> | |||
{ config, lib, ... }: | |||
< | # in any nix code: | ||
lib.warn "This is a sample warning message." | |||
{ | |||
config.warnings = ( | |||
# Some NixOS module: throw error, if services.foo.bar == true | |||
lib.optionals config.services.foo.bar "This is also a sample warning message, but invoked differently." | |||
); | |||
} | |||
</syntaxHighlight> | |||
== Nix == | == Nix == | ||
Line 11: | Line 19: | ||
The nix language has a construct to help with printing messages. | The nix language has a construct to help with printing messages. | ||
* '''assert''': throw an error (see [https://nixos.org/manual/nix/stable/language/constructs.html?highlight=assert#assertions | * '''assert''': throw an error (see [https://nixos.org/manual/nix/stable/language/constructs.html?highlight=assert#assertions Nix manual: Assertions]) | ||
The nix language also comes with some related [https://nixos.org/manual/nix/stable/language/builtins.html | 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 | ||
Line 22: | Line 30: | ||
* '''tryEval''': catch throws and asserts | * '''tryEval''': catch throws and asserts | ||
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 | Most of those functions (nix builtins as well as nixpkgs lib functions) take an expression <code>e</code> as their last argument which they return unmodified. Thus they are chained in front of some expression: | ||
<syntaxHighlight lang=nix> | |||
a = builtins.trace "trace message" { | |||
# what should be assigned to a | |||
}; | |||
</syntaxHighlight> | |||
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 == | ||
Line 28: | Line 44: | ||
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.* (see [https://nixos.org/manual/nixpkgs/stable/#sec-functions-library-trivial | * lib.trivial.* (see [https://nixos.org/manual/nixpkgs/stable/#sec-functions-library-trivial nixpkgs manual: lib.trivial]) | ||
** lib.'''throwIf''' and throwIfNot | ** lib.'''throwIf''' and throwIfNot | ||
** lib.'''warn''', '''warnIf''' and warnIfNot | ** lib.'''warn''', '''warnIf''' and warnIfNot | ||
* [https://nixos.org/manual/nixpkgs/stable/#sec-functions-library-debug | * [https://nixos.org/manual/nixpkgs/stable/#sec-functions-library-debug lib.'''debug'''.*]: tracing functions with some pretty printing (e.g. <code>lib.debug.traceIf</code>) <ref>[http://ryantm.github.io/nixpkgs/functions/library/debug/#sec-functions-library-debug Nixpkgs/docs: lib.debug]</ref> | ||
* [https://nixos.org/manual/nixpkgs/stable/#sec-functions-library-asserts | * [https://nixos.org/manual/nixpkgs/stable/#sec-functions-library-asserts lib.'''asserts'''.*]: assert functions (e.g. <code>lib.asserts.assertMsg</code>) | ||
These facilities also expose their attributes directly via <code>lib.*</code> (e.g. <code>lib.throwIf</code>). | 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 <code>break</code>: the [https://nixos.org/manual/nixpkgs/stable/#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 (see [https://nixos.org/manual/nixos/stable/index.html#sec-assertions-warnings | 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> | ||
* <code>config.warnings = [];</code> | * <code>config.warnings = [];</code> | ||
* <code>config.assertions = [];</code> | * <code>config.assertions = [];</code> | ||
An example for a debugging facility in NixOS is running [https://nixos.org/manual/nixos/stable/index.html#sec-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 == | ||
Line 51: | Line 67: | ||
<!-- put debugging into a heading for search engine optimisation --> | <!-- put debugging into a heading for search engine optimisation --> | ||
To summarise debugging approaches discussed in this article, you can use <code>break</code> to debug nix code, <code>breakpointHook</code> to debug nix builds and interactive tools to debug NixOS tests. | |||
To find the location where variables get defined, you can use the following tools: | |||
For bare nix code, use <code>builtins.unsafeGetAttrPos | |||
</code> ([https://github.com/NixOS/nix/blob/b17c4290cf61d8a0386817b87231762c175097c5/tests/lang/eval-okay-getattrpos.nix example]) which returns the line and column of where an attribute is defined. It is undocumented and considered bad practice. | |||
(soon to come [https://github.com/NixOS/nixpkgs/pull/249243 github PR]): For NixOS options unsafeGetAttrPos doesn't work, but the module system itself records that information: to find the location of <code>config.networking.hostName</code>, use <code>:p options.networking.hostName.declarationPositions</code>. | |||
== References == | == References == |
Latest revision as of 12:39, 19 January 2024
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.
In most cases you will want to stick to the highest level abstraction: config.warnings
or lib.warn
and its relatives.
{ config, lib, ... }:
# in any nix code:
lib.warn "This is a sample warning message."
{
config.warnings = (
# Some NixOS module: throw error, if services.foo.bar == true
lib.optionals config.services.foo.bar "This is also a sample warning message, but invoked differently."
);
}
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
Most of those functions (nix builtins as well as nixpkgs lib functions) take an expression e
as their last argument which they return unmodified. Thus they are chained in front of some expression:
a = builtins.trace "trace message" {
# what should be assigned to a
};
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). [1]
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 (e.g.
lib.debug.traceIf
) [2] - lib.asserts.*: assert functions (e.g.
lib.asserts.assertMsg
)
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): [3]
config.warnings = [];
config.assertions = [];
An example for a debugging facility in NixOS is running NixOS tests interactively.
Debugging
To summarise debugging approaches discussed in this article, you can use break
to debug nix code, breakpointHook
to debug nix builds and interactive tools to debug NixOS tests.
To find the location where variables get defined, you can use the following tools:
For bare nix code, use builtins.unsafeGetAttrPos
(example) which returns the line and column of where an attribute is defined. It is undocumented and considered bad practice.
(soon to come github PR): For NixOS options unsafeGetAttrPos doesn't work, but the module system itself records that information: to find the location of config.networking.hostName
, use :p options.networking.hostName.declarationPositions
.