72 lines
2.3 KiB
Markdown
72 lines
2.3 KiB
Markdown
---
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title: Extensible configuration Pt. 1
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date: 2019-03-27
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---
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This is the first part of a series where I'm going through how to make
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extensible configuration. There is nothing groundbreaking or new in this
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series, it's just me going through different implementations and trying to
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understand them.
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The source material for this post is [the fixed point implementation](https://github.com/NixOS/nixpkgs/blob/master/lib/fixed-points.nix) for nix.
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By extensible configuration, I'm talking about nix style extensible
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configuration, like overlays, overrides and extensions. Let's see an example of
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an extensible configuration.
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``` nix
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{ haskellPackages, fetchFromGitHub }:
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let
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purescript = fetchFromGitHub {
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owner = "purescript";
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repo = "purescript";
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rev = "2cb4a6496052db726e099539be682b87585af494";
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sha256 = "1v4gs08xnqgym6jj3drkzbic7ln3hfmflpbpij3qzwxsmqd2abr7";
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}
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hp = haskellPackages.extend (self: super: {
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purescript = super.callCabal2nix "purescript" purescript {};
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});
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in
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hp.purescript;
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```
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On a high level we are augmenting the `haskellPackages` attrset by replacing
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the existing purescript package with a different one. The extension is a
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function that takes two arguments, `self` and `super`. `super` is the original
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non-lazy value and `self` is the lazy value that corresponds to the value at
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end.
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The first step on this journey is done by getting to know `fix`. Fix is
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described being the least fixed point of a function. In practice it's a
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function allowing declaring recursive functions without explicit recursion.
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``` nix
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fix = f: let x = f x; in x
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```
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With fix you can have access to the lazy `self` value. It's value is whatever
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would have been computed in the end. As it is lazy, it is possible to end up in
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a recursive loop if there is a cyclic dependency.
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``` nix
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let recursive = fix (self: {
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foo = 3;
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bar = self.foo + 1;
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});
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infinite = fix (self: {
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foo = self.bar + 1;
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bar = self.foo + 1;
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});
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```
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You can try those yourself. The first version is fine and returns an attrset
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like you would expect. The second one has a cyclic dependency and nix helpfully
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errors out.
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- https://elvishjerricco.github.io/2017/04/01/nix-style-configs-in-haskell.html
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- https://github.com/NixOS/nixpkgs/blob/master/lib/fixed-points.nix
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- https://chshersh.github.io/posts/2019-03-25-comonadic-builders
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