build: flake template from the-nix-way

flake.nix

@@ -0,0 +1,39 @@
+{
+  description = "A Nix-flake-based Haskell development environment";
+
+  inputs.nixpkgs.url = "https://flakehub.com/f/NixOS/nixpkgs/0.1";
+
+  outputs =
+    inputs:
+    let
+      supportedSystems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forEachSupportedSystem =
+        f:
+        inputs.nixpkgs.lib.genAttrs supportedSystems (
+          system:
+          f {
+            pkgs = import inputs.nixpkgs { inherit system; };
+          }
+        );
+    in
+    {
+      devShells = forEachSupportedSystem (
+        { pkgs }:
+        {
+          default = pkgs.mkShell {
+            packages = with pkgs; [
+              cabal-install
+              ghc
+              haskell-language-server
+              just
+            ];
+          };
+        }
+      );
+    };
+}