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| 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:
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| # creates a derivation which will build a Nix expression
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| # imports that expression
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| # uses the results of the evaluation of the expression.
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| An example of IFD is:
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| <syntaxHighlight lang=nix>
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| let
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| pkgs = import <nixpkgs> {};
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| # Create a derivation which, when built, writes some Nix code to
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| # its $out path.
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| derivation-to-import = pkgs.writeText "example" ''
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| pkgs: {
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| ifd-example = pkgs.stdenv.mkDerivation rec {
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| name = "hello-2.10-ifd-example";
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| src = pkgs.fetchurl {
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| url = "mirror://gnu/hello/2.10.tar.gz";
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| sha256 = "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i";
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| };
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| };
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| }
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| '';
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| # Import the derivation. This forces `derivation-to-import` to become
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| # a string. This is normal behavior for Nix and Nixpkgs. The specific
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| # difference here is the evaluation itself requires the result to be
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| # built during the evaluation in order to continue evaluating.
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| imported-derivation = import derivation-to-import;
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| # Treat the imported-derivation variable as if we hadn't just created
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| # its Nix expression inside this same evaluation.
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| hello-package = (imported-derivation pkgs).ifd-example;
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| in hello-package
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| </syntaxHighlight>
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| Building this looks familiar, but with an extra <code>building ...</code> line:
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| {{Commands|<nowiki>
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| $ nix-build ./test.nix
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| building '/nix/store/8n001pyx2iqsnzd6niji1bvyjlg6x058-example.drv'... <- this build is forced at
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| evaluation time
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| these derivations will be built: <- now we're back to normal
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| nix-build behavior
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| /nix/store/3nm9rlv5smmvijcdifngjwl4v6zvll7k-hello-2.10-ifd-example.drv
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| building '/nix/store/3nm9rlv5smmvijcdifngjwl4v6zvll7k-hello-2.10-ifd-example.drv'...
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| [...snip...] | |
| </nowiki>}}
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| we'll see pretty similar output if we just evaluate it:
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| {{Commands|<nowiki>
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| $ nix-instantiate ./test.nix
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| building '/nix/store/8n001pyx2iqsnzd6niji1bvyjlg6x058-example.drv'...
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| /nix/store/3nm9rlv5smmvijcdifngjwl4v6zvll7k-hello-2.10-ifd-example.drv
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| </nowiki>}}
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| Some examples of IFD can be seen when using nixpkgs to fetch a specific
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| version of nixpkgs, and then importing the source.
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| == When to use IFD ==
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| 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.
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| 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.
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| 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.
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| == Recognizing IFD ==
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| 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.
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| 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
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| * <code>import (f drv)</code>
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| * <code>builtins.readFile (f drv)</code>
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| * <code>builtins.readDir (f drv)</code>
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| * <code>builtins.pathExists (f drv)</code>
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| * <code>builtins.filterSource g (f drv) </code>
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| * <code>builtins.path { path = f drv; }</code>
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| * <code>builtins.hashFile t (f drv)</code>
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| * <code>builtins.scopedImport x (f drv)</code>
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| * potentially other functions if those read the filesystem in any way
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| * '''library functions''' that invoke the above, such as
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| * <code>lib.cleanSource { src = f drv; }</code>
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| * ...
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| == IFD and Haskell ==
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| <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:
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| * <code>nix flake show</code>
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| * <code>nix flake check</code>
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|
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| == Further reading == | | == Further reading == |
|
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| * https://fzakaria.com/2020/10/21/nix-parallelism-import-from-derivation.html | | * https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html |
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| [[Category: Nix]] | | [[Category: Nix]] |
