Add notes on nix.

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+    "nix/callPackage.md": "5ef6bc5d1a549c55d43ebb4d48c64427",
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notes/nix/callPackage.md

@@ -0,0 +1,116 @@
+---
+title: callPackage
+TARGET DECK: Obsidian::STEM
+FILE TAGS: nix
+tags:
+  - nix
+---
+
+## Overview
+
+We first examine `lib.makeOverridable`. It's implementation isn't too important but understanding how it's used is. We adapt the example found in [nixpkgs](https://github.com/NixOS/nixpkgs/blob/56df668386ac83c5bcddf9849c645cf0d25706d7/lib/customisation.nix#L77):
+
+```nix
+nix-repl> x = {a, b}: { example = a + b; }
+nix-repl> y = lib.makeOverridable x { a = 1; b = 2; }
+nix-repl> y
+{ override = «lambda»; overrideDerivation = «lambda»; example = 3; }
+nix-repl> y.override { a = 10; }
+{ override = «lambda»; overrideDerivation = «lambda»; example = 12; }
+```
+
+`lib.makeOverridable` is an example of partial function application. Notice `y` still contains the result (`example`) of evaluating `x`. We can re-run the computation with a different value (in this case `a`) by invoking `y.override`.
+
+%%ANKI
+Basic
+What is the result of the following?
+```nix
+x = {a, b}: { example = a + b; }
+lib.functionArgs x
+```
+Back: `{ a = false; b = false; }`
+Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. [https://github.com/ryan4yin/nixos-and-flakes-book](https://github.com/ryan4yin/nixos-and-flakes-book)
+<!--ID: 1706828138583-->
+END%%
+
+%%ANKI
+Basic
+What does each boolean returned by `lib.functionArgs` indicate?
+Back: Whether the associated attribute has a default value.
+Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. [https://github.com/ryan4yin/nixos-and-flakes-book](https://github.com/ryan4yin/nixos-and-flakes-book)
+<!--ID: 1706828138588-->
+END%%
+
+%%ANKI
+Basic
+What additional attributes is included in the set returned by `lib.makeOverridable`?
+Back: `override` and `overrideDerivation`.
+Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. [https://github.com/ryan4yin/nixos-and-flakes-book](https://github.com/ryan4yin/nixos-and-flakes-book)
+<!--ID: 1706828138590-->
+END%%
+
+%%ANKI
+Basic
+What is the value of `y.example` in the following?
+```nix
+x = {a, b}: { example = a + b; }
+y = lib.makeOverridable x { a = 1; b = 2; }
+```
+Back: `3`
+Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. [https://github.com/ryan4yin/nixos-and-flakes-book](https://github.com/ryan4yin/nixos-and-flakes-book)
+<!--ID: 1706828225233-->
+END%%
+
+%%ANKI
+Basic
+What is the value of `(y.override { a = 10; }).example` in the following?
+```nix
+x = {a, b}: { example = a + b; }
+y = lib.makeOverridable x { a = 1; b = 2; }
+```
+Back: `12`
+Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. [https://github.com/ryan4yin/nixos-and-flakes-book](https://github.com/ryan4yin/nixos-and-flakes-book)
+<!--ID: 1706828225236-->
+END%%
+
+Now we can understand how `pkgs.callPackage` works. The following is a simplification of the [actual implementation](https://github.com/NixOS/nixpkgs/blob/56df668386ac83c5bcddf9849c645cf0d25706d7/lib/customisation.nix#L153):
+
+```nix
+callPackageWith = autoArgs: fn: args:
+  let
+    f = if isFunction fn then fn else import fn;
+    fargs = functionArgs f;
+    allArgs = intersectArgs fargs autoArgs // args;
+  in
+    lib.makeOverridable f allArgs
+
+callPackage = callPackageWith pkgs;
+```
+
+%%ANKI
+Basic
+What two functions is `callPackage` implemented on top of?
+Back: `callPackageWith` and `lib.makeOverridable`.
+Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. [https://github.com/ryan4yin/nixos-and-flakes-book](https://github.com/ryan4yin/nixos-and-flakes-book)
+<!--ID: 1706828138592-->
+END%%
+
+%%ANKI
+Basic
+What is the purpose of `callPackage`?
+Back: It calls package functions with arguments automatic supplied if not overridden. 
+Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. [https://github.com/ryan4yin/nixos-and-flakes-book](https://github.com/ryan4yin/nixos-and-flakes-book)
+<!--ID: 1706828138594-->
+END%%
+
+%%ANKI
+Basic
+What attribute must be invoked in `callPackage`'s return value to override arguments?
+Back: `override`
+Reference: Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. [https://github.com/ryan4yin/nixos-and-flakes-book](https://github.com/ryan4yin/nixos-and-flakes-book)
+<!--ID: 1706828225240-->
+END%%
+
+## Reference
+
+* Yin, Ryan. “NixOS and Flakes Book.” Nix, February 1, 2024. [https://github.com/ryan4yin/nixos-and-flakes-book](https://github.com/ryan4yin/nixos-and-flakes-book)

notes/nix/index.md

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+---
+title: Nix
+---