Tooling: Move from nix-shell to flakes

.gitignore

@@ -1 +1,2 @@
 data/
+result

README.md

@@ -7,7 +7,7 @@ An ingress for RSS, Atom, JSONFeed, and other news feeds that provides configura
 ### Local Go
 
 ```shell
-$ nix-shell # Or local environment with go + docker-compose
+$ nix develop # Or local environment with go + docker-compose
 $ $EDITOR config.yaml
 $ docker-compose -f docker-compose.dependencies.yml up -d
 $ go run *.go

flake.nix

@@ -0,0 +1,71 @@
+{
+  description = "Simple configurable RSS ingress";
+
+  # Nixpkgs / NixOS version to use.
+  inputs.nixpkgs.url = "nixpkgs/nixos-21.11";
+
+  outputs = { self, nixpkgs }:
+    let
+
+      # Generate a user-friendly version number.
+      version = builtins.substring 0 8 self.lastModifiedDate;
+
+      # System types to support.
+      supportedSystems = [ "x86_64-linux" "x86_64-darwin" "aarch64-linux" "aarch64-darwin" ];
+
+      # 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
+    {
+
+      # Provide some binary packages for selected system types.
+      packages = forAllSystems (system:
+        let
+          pkgs = nixpkgsFor.${system};
+        in
+        {
+          log2http = pkgs.buildGoModule {
+            pname = "feed-ingress";
+            inherit version;
+            # In 'nix develop', we don't need a copy of the source tree
+            # in the Nix store.
+            src = ./.;
+
+            # This hash locks the dependencies of this package. It is
+            # necessary because of how Go requires network access to resolve
+            # VCS.  See https://www.tweag.io/blog/2021-03-04-gomod2nix/ for
+            # details. Normally one can build with a fake sha256 and rely on native Go
+            # mechanisms to tell you what the hash should be or determine what
+            # it should be "out-of-band" with other tooling (eg. gomod2nix).
+            # To begin with it is recommended to set this, but one must
+            # remeber to bump this hash when your dependencies change.
+            # vendorSha256 = pkgs.lib.fakeSha256;
+
+            vendorSha256 = "sha256-cfBzHO7b2fNUWrlls/FcgfmbHjvOGugcQbd+EDiUmg0=";
+          };
+        });
+
+      # The default package for 'nix build'. This makes sense if the
+      # flake provides only one package or there is a clear "main"
+      # package.
+      defaultPackage = forAllSystems (system: self.packages.${system}.log2http);
+
+      devShell = forAllSystems (system:
+        let pkgs = nixpkgsFor.${system};
+        in pkgs.mkShell {
+          buildInputs = with pkgs; [
+            docker-compose
+            go
+            gopls
+            goimports
+            go-tools
+            kcat
+          ];
+        });
+
+    };
+}