Error handling: Difference between revisions

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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.   


{{Expansion|This page is currently being worked on (04.05.2023) and desperately needs usage examples.}}
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.  


In most cases you will want to stick to the highest level abstraction: config.warnings or lib.assertMsg
<syntaxHighlight lang=nix>
 
{ config, lib, ... }:  
<give examples for both>
# 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 ==
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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; Nix manual: 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; builtin functions]:
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
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* '''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; throw vs assert discussion]</ref>
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 ==
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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.* (see [https://nixos.org/manual/nixpkgs/stable/#sec-functions-library-trivial; nixpkgs manual: lib.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; 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>
* [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; lib.'''asserts'''.*]: assert functions
* [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; 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; 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>
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; 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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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.  
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.

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:

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.

References