posts/2024-12-19/01: add "Quick bits: basic flake.nix template"

posts/2024-12-19/01-quick-bits-basic-flake-nix-template.md

@@ -0,0 +1,50 @@
+# Quick bits: basic flake.nix template
+
+Sometimes I want a really basic `flake.nix` that has no dependencies except for
+`nixpkgs` itself, e.g.: I want to avoid
+[flake-utils](https://github.com/numtide/flake-utils) or any other dependency.
+So, here you go:
+
+```nix
+{
+  description = "Description";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  };
+
+  outputs =
+    { self, nixpkgs, ... }:
+    let
+      supportedSystems = [
+        "aarch64-linux"
+        "x86_64-linux"
+        "aarch64-darwin"
+        "x86_64-linux"
+      ];
+
+      # Helper function to generate an attrset '{ x86_64-linux = f "x86_64-linux"; ... }'.
+      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
+
+      # Nixpkgs instantiated for supported system types.
+      nixpkgsFor = forAllSystems (system: import nixpkgs { inherit system; });
+    in
+    {
+      packages = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgsFor.${system};
+        in
+        rec {
+          default = pkgs.hello;
+        }
+      );
+    };
+}
+```
+
+Not sure where I grabbed the definition for `forAllSystems` and `nixpkgsFor`. I
+have the impression it was in a [Julia Evans blog post](https://jvns.ca/), but
+I can't find it.
+
+Anyway, it is here for me to remember, and it may help someone else.