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| Import From Derivation 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
| | == Further reading == |
| # 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:
| | * https://fzakaria.com/2020/10/20/nix-parallelism-import-from-derivation.html |
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| <syntaxHighlight lang=nix>
| | [[Category: Nix]] |
| 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...]
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| </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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