75 lines
2.9 KiB
Markdown
75 lines
2.9 KiB
Markdown
# Fixed-point arguments of build helpers {#chap-build-helpers-finalAttrs}
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As mentioned in the beginning of this part, `stdenv.mkDerivation` could alternatively accept a fixed-point function. The input of such function, typically named `finalAttrs`, is expected to be the final state of the attribute set.
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A build helper like this is said to accept **fixed-point arguments**.
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Build helpers don't always support fixed-point arguments yet, as support in [`stdenv.mkDerivation`](#mkderivation-recursive-attributes) was first included in Nixpkgs 22.05.
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## Defining a build helper with `lib.extendMkDerivation` {#sec-build-helper-extendMkDerivation}
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Developers can use the Nixpkgs library function [`lib.customisation.extendMkDerivation`](#function-library-lib.customisation.extendMkDerivation) to define a build helper supporting fixed-point arguments from an existing one with such support, with an attribute overlay similar to the one taken by [`<pkg>.overrideAttrs`](#sec-pkg-overrideAttrs).
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Beside overriding, `lib.extendMkDerivation` also supports `excludeDrvArgNames` to optionally exclude some arguments in the input fixed-point arguments from passing down the base build helper (specified as `constructDrv`).
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:::{.example #ex-build-helpers-extendMkDerivation}
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# Example definition of `mkLocalDerivation` extended from `stdenv.mkDerivation` with `lib.extendMkDerivation`
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We want to define a build helper named `mkLocalDerivation` that builds locally without using substitutes by default.
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Instead of taking a plain attribute set,
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```nix
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{
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preferLocalBuild ? true,
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allowSubstitute ? false,
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specialArg ? (_: false),
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...
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}@args:
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stdenv.mkDerivation (
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removeAttrs [
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# Don't pass specialArg into mkDerivation.
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"specialArg"
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] args
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// {
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# Arguments to pass
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inherit preferLocalBuild allowSubstitute;
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# Some expressions involving specialArg
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greeting = if specialArg "hi" then "hi" else "hello";
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}
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)
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```
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we could define with `lib.extendMkDerivation` an attribute overlay to make the result build helper also accepts the the attribute set's fixed point passing to the underlying `stdenv.mkDerivation`, named `finalAttrs` here:
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```nix
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lib.extendMkDerivation {
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constructDrv = stdenv.mkDerivation;
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excludeDrvArgNames = [
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# Don't pass specialArg into mkDerivation.
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"specialArg"
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];
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extendDrvArgs =
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finalAttrs:
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{
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preferLocalBuild ? true,
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allowSubstitute ? false,
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specialArg ? (_: false),
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...
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}@args:
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{
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# Arguments to pass
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inherit
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preferLocalBuild
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allowSubstitute
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;
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# Some expressions involving specialArg
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greeting = if specialArg "hi" then "hi" else "hello";
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};
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}
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```
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:::
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If one needs to apply extra changes to the result derivation, pass the derivation transformation function to `lib.extendMkDerivation` as `lib.customisation.extendMkDerivation { transformDrv = drv: ...; }`.
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