wip

posts/configs.md

@@ -0,0 +1,71 @@
+---
+title: Extensible configuration Pt. 1
+date: 2019-03-27
+---
+
+This is the first part of a series where I'm going through how to make
+extensible configuration. There is nothing groundbreaking or new in this
+series, it's just me going through different implementations and trying to
+understand them.
+
+The source material for this post is [the fixed point implementation](https://github.com/NixOS/nixpkgs/blob/master/lib/fixed-points.nix) for nix.
+
+By extensible configuration, I'm talking about nix style extensible
+configuration, like overlays, overrides and extensions. Let's see an example of
+an extensible configuration.
+
+``` nix
+{ haskellPackages, fetchFromGitHub }:
+
+let
+  purescript = fetchFromGitHub {
+    owner = "purescript";
+    repo = "purescript";
+    rev = "2cb4a6496052db726e099539be682b87585af494";
+    sha256 = "1v4gs08xnqgym6jj3drkzbic7ln3hfmflpbpij3qzwxsmqd2abr7";
+  }
+  hp = haskellPackages.extend (self: super: {
+    purescript = super.callCabal2nix "purescript" purescript {};
+  });
+
+in
+
+hp.purescript;
+```
+
+On a high level we are augmenting the `haskellPackages` attrset by replacing
+the existing purescript package with a different one. The extension is a
+function that takes two arguments, `self` and `super`. `super` is the original
+non-lazy value and `self` is the lazy value that corresponds to the value at
+end.
+
+The first step on this journey is done by getting to know `fix`. Fix is
+described being the least fixed point of a function. In practice it's a
+function allowing declaring recursive functions without explicit recursion.
+
+``` nix
+fix = f: let x = f x; in x
+```
+
+With fix you can have access to the lazy `self` value. It's value is whatever
+would have been computed in the end. As it is lazy, it is possible to end up in
+a recursive loop if there is a cyclic dependency.
+
+``` nix
+let recursive = fix (self: {
+  foo = 3;
+  bar = self.foo + 1;
+});
+infinite = fix (self: {
+  foo = self.bar + 1;
+  bar = self.foo + 1;
+});
+```
+
+You can try those yourself. The first version is fine and returns an attrset
+like you would expect. The second one has a cyclic dependency and nix helpfully
+errors out.
+
+- https://elvishjerricco.github.io/2017/04/01/nix-style-configs-in-haskell.html
+- https://github.com/NixOS/nixpkgs/blob/master/lib/fixed-points.nix
+- https://chshersh.github.io/posts/2019-03-25-comonadic-builders