Import From Derivation - Revision history

N0099 at 21:07, 13 January 2026

2026-01-13T21:07:36Z

← Older revision		Revision as of 21:07, 13 January 2026
Line 3:		Line 3:
	== Further reading ==		== Further reading ==

	* https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html		* [https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html Nix parallelism & Import From Derivation \| Farid Zakaria's Blog]
	* [https://nixcademy.com/posts/what-is-ifd-ups-and-downs/ Nix IFD: A Ticking Time Bomb in Your Build Pipeline? Nixcademy]		* [https://nixcademy.com/posts/what-is-ifd-ups-and-downs/ Nix IFD: A Ticking Time Bomb in Your Build Pipeline? Nixcademy]
			* [https://jade.fyi/blog/nix-evaluation-blocking/ Stopping evaluation from blocking in Nix - jade's www site]

	[[Category: Nix]]		[[Category: Nix]]

Mightyiam: Link to Nixcademy blog post on IFD

2025-12-26T06:38:04Z

Link to Nixcademy blog post on IFD

← Older revision		Revision as of 06:38, 26 December 2025
Line 4:		Line 4:

	* https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html		* https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html
			* [https://nixcademy.com/posts/what-is-ifd-ups-and-downs/ Nix IFD: A Ticking Time Bomb in Your Build Pipeline? Nixcademy]

	[[Category: Nix]]		[[Category: Nix]]

Mic92 at 09:57, 13 September 2024

2024-09-13T09:57:45Z

← Older revision		Revision as of 09:57, 13 September 2024
Line 3:		Line 3:
	== Further reading ==		== Further reading ==

	* https://fzakaria.com/2020/10/21/nix-parallelism-import-from-derivation.html		* https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html

	[[Category: Nix]]		[[Category: Nix]]

imported>Fricklerhandwerk: IFD is now documented in the Nix manual

2023-10-09T21:54:31Z

IFD is now documented in the Nix manual

← Older revision		Revision as of 21:54, 9 October 2023
Line 1:		Line 1:
	~~Import From Derivation (IFD) is where during a single Nix evaluation, the~~		See the Nix manual: [https://nixos.org/manual/nix/unstable/language/import-from-derivation Import From Derivation]
	~~Nix expression:~~

	~~# creates a derivation which will build a Nix expression~~
	~~# imports that expression~~
	~~# uses the results of the evaluation of~~ the ~~expression.~~

	~~An example of IFD is:~~

	~~<syntaxHighlight lang=nix>~~
	~~let~~
	~~pkgs = import <nixpkgs> {};~~

	~~# Create a derivation which, when built, writes some~~ Nix ~~code to~~
	~~# its $out path.~~
	~~derivation-to-import = pkgs.writeText "example" ''~~
	~~pkgs: {~~
	~~ifd-example = pkgs.stdenv.mkDerivation rec {~~
	~~name = "hello-2.10-ifd-example";~~


	~~src = pkgs.fetchurl {~~
	~~url = "mirror~~:~~//gnu/hello/2.10.tar.gz";~~
	~~sha256 = "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i";~~
	};
	};
	}
	~~'';~~

	~~# Import the derivation. This forces `derivation-to-import` to become~~
	~~# a string. This is normal behavior for Nix and Nixpkgs. The specific~~
	~~# difference here is the evaluation itself requires the result to be~~
	~~# built during the evaluation in order to continue evaluating.~~
	~~imported-derivation = import derivation-to-import;~~

	~~# Treat the imported-derivation variable as if we hadn't just created~~
	~~# its Nix expression inside this same evaluation.~~
	~~hello-package = (imported-derivation pkgs).ifd-example;~~
	~~in hello-package~~
	~~</syntaxHighlight>~~

	~~Building this looks familiar, but with an extra <code>building ...</code> line:~~

	~~{{Commands\|<nowiki>~~
	~~$ nix-build ./test.nix~~
	~~building '/nix/store/8n001pyx2iqsnzd6niji1bvyjlg6x058-example.drv'... <- this build is forced at~~
	~~evaluation time~~

	~~these derivations will be built: <- now we're back to normal~~
	~~nix-build behavior~~
	~~/nix/store/3nm9rlv5smmvijcdifngjwl4v6zvll7k-hello-2.10-ifd-example.drv~~

	~~building '/nix/store/3nm9rlv5smmvijcdifngjwl4v6zvll7k-hello-2.10-ifd-example.drv'...~~
	[~~...snip...]~~
	~~</nowiki>}}~~

	~~we'll see pretty similar output if we just evaluate it~~:

	~~{{Commands\|<nowiki>~~
	~~$ nix-instantiate .~~/~~test.nix~~
	~~building '~~/~~nix/store/8n001pyx2iqsnzd6niji1bvyjlg6x058-example~~.~~drv'...~~

	~~/nix/store~~/~~3nm9rlv5smmvijcdifngjwl4v6zvll7k-hello-2.10-ifd-example.drv~~
	</~~nowiki>}}~~

	~~Some examples of IFD can be seen when using nixpkgs to fetch a specific~~
	~~version of nixpkgs, and then importing the source.~~

	~~== When to use IFD ==~~

	~~IFD is a powerful feature, but it should be avoided _if possible_. IFD extends the duration of evaluation, which is single threaded and it affects the predictions of the `~~nix~~` progress bar.~~

	~~Acceptable uses of IFD include importing a pinned nixpkgs and automation around lock files. Such uses vastly improve your development workflow, outweighing the slight disadvantages of IFD.~~

	~~As a rule of thumb, if you can avoid IFD by refactoring Nix code or moving your build logic into the derivations themselves, you should do so.~~

	~~== Recognizing IFD ==~~

	~~The textbook case of IFD looks like <code>import (f drv)</code> where <code>f drv<~~/~~code> is an expression that requires a build in order to return a value. However, <code>import<~~/~~code> is not the only function that can require a build. For example, <code>builtins.readFile<~~/~~code> can have the same effect.~~

	~~All functions that query the filesystem will require a build when invoked on a derivation output path or subpath. This has the same effect and is therefore also called IFD. These include~~

	* <code>import ~~(f drv)</code>~~
	* <code>builtins.readFile (f drv)</code>
	* <code>builtins.readDir (f drv)</code>
	* <code>builtins.pathExists (f drv)</code>
	* <code>builtins.filterSource g (f drv) </code>
	* <code>builtins.path { path = f drv; }</code>
	* <code>builtins.hashFile t (f drv)</code>
	* <code>builtins.scopedImport x (f drv)</code>
	* potentially other functions if those read the filesystem in any way
	* '''library functions''' that invoke the above, such as
	* <code>lib.cleanSource { src = f drv; }</code>
	* ...

	~~== IFD and Haskell ==~~

	~~<code>callCabal2nix</code>, which is implicitly used for building Haskell projects, uses IFD.[https://github.com/NixOS/templates/issues/28][https://discourse.nixos.org/t/another~~-~~simple~~-~~flake-for-haskell-development/18164/6~~]. This means that since IFD is disabled by default in certain nix commands,[https://github.com/NixOS/nix/pull/5253] the following commands will be broken for Haskell projects whose flake output specifies multiple system attributes:

	* <code>nix flake show</code>
	* <code>nix flake check</code>

	== Further reading ==		== Further reading ==

imported>Bpfoley: Typo in URL :)

2022-10-11T19:53:23Z

Typo in URL :)

← Older revision		Revision as of 19:53, 11 October 2022
Line 103:		Line 103:
	== Further reading ==		== Further reading ==

	* https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html		* https://fzakaria.com/2020/10/21/nix-parallelism-import-from-derivation.html

	[[Category: Nix]]		[[Category: Nix]]

imported>Fufexan: Add page to Nix category

2022-08-09T20:55:56Z

Add page to Nix category

← Older revision		Revision as of 20:55, 9 August 2022
Line 104:		Line 104:

	* https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html		* https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html

			[[Category: Nix]]

imported>Srid: IFD and haskell

2022-03-26T18:45:52Z

IFD and haskell

← Older revision		Revision as of 18:45, 26 March 2022
Line 93:		Line 93:
	* <code>lib.cleanSource { src = f drv; }</code>		* <code>lib.cleanSource { src = f drv; }</code>
	* ...		* ...

			== IFD and Haskell ==

			<code>callCabal2nix</code>, which is implicitly used for building Haskell projects, uses IFD.[https://github.com/NixOS/templates/issues/28][https://discourse.nixos.org/t/another-simple-flake-for-haskell-development/18164/6]. This means that since IFD is disabled by default in certain nix commands,[https://github.com/NixOS/nix/pull/5253] the following commands will be broken for Haskell projects whose flake output specifies multiple system attributes:

			* <code>nix flake show</code>
			* <code>nix flake check</code>

	== Further reading ==		== Further reading ==

	* https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html		* https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html

imported>Asymmetric: Undo revision 6945 by Asymmetric (talk)

2021-11-08T18:04:27Z

Undo revision 6945 by Asymmetric (talk)

← Older revision		Revision as of 18:04, 8 November 2021
Line 29:		Line 29:

	# Import the derivation. This forces `derivation-to-import` to become		# Import the derivation. This forces `derivation-to-import` to become
	# a string. This is normal behavior for Nix and Nixpkgs. The ~~string is~~		# a string. This is normal behavior for Nix and Nixpkgs. The specific
	# ~~then evaluated, which in this case triggers a build, which blocks~~		# difference here is the evaluation itself requires the result to be
	# the ~~single-threaded~~ evaluation.		# built during the evaluation in order to continue evaluating.
	imported-derivation = import derivation-to-import;		imported-derivation = import derivation-to-import;

imported>Asymmetric: Clarify example documentation

2021-11-08T17:46:12Z

Clarify example documentation

← Older revision		Revision as of 17:46, 8 November 2021
Line 29:		Line 29:

	# Import the derivation. This forces `derivation-to-import` to become		# Import the derivation. This forces `derivation-to-import` to become
	# a string. This is normal behavior for Nix and Nixpkgs. The ~~specific~~		# a string. This is normal behavior for Nix and Nixpkgs. The string is
	# ~~difference here is the evaluation itself requires the result to be~~		# then evaluated, which in this case triggers a build, which blocks
	# ~~built during~~ the evaluation ~~in order to continue evaluating~~.		# the single-threaded evaluation.
	imported-derivation = import derivation-to-import;		imported-derivation = import derivation-to-import;

imported>Asymmetric: fix code syntax

2021-11-08T17:02:34Z

fix code syntax

← Older revision		Revision as of 17:02, 8 November 2021
Line 77:		Line 77:
	== Recognizing IFD ==		== Recognizing IFD ==

	The textbook case of IFD looks like `import (f drv)` where `f drv` is an expression that requires a build in order to return a value. However, `import` is not the only function that can require a build. For example, `builtins.readFile` can have the same effect.		The textbook case of IFD looks like <code>import (f drv)</code> where <code>f drv</code> is an expression that requires a build in order to return a value. However, <code>import</code> is not the only function that can require a build. For example, <code>builtins.readFile</code> can have the same effect.

	All functions that query the filesystem will require a build when invoked on a derivation output path or subpath. This has the same effect and is therefore also called IFD. These include		All functions that query the filesystem will require a build when invoked on a derivation output path or subpath. This has the same effect and is therefore also called IFD. These include