Add Nix flake

flake.nix

@@ -0,0 +1,47 @@
+{
+  description = "Simple pool implementation";
+
+  inputs = {
+    flake-utils.url = github:numtide/flake-utils;
+  };
+
+  outputs = { self, nixpkgs, flake-utils }: {
+    overlay = final: prev: {
+      haskell = prev.haskell.override {
+        packageOverrides = final: prev: {
+          simpoole = prev.callCabal2nix "simpoole" self { };
+        };
+      };
+    };
+  } // flake-utils.lib.eachDefaultSystem (system:
+    with (import nixpkgs {
+      inherit system;
+      config = { overlays = [ self.overlay ]; };
+    });
+
+    {
+      packages = {
+        simpoole = haskellPackages.simpoole;
+      };
+
+      defaultPackage = self.packages.${system}.simpoole;
+
+      checks = {
+        simpoole = self.packages.${system}.simpoole.overrideAttrs (_: { doCheck = true; });
+      };
+
+      devShell = mkShell {
+        buildInputs = [
+          (haskellPackages.ghcWithPackages (hspkgs:
+            hspkgs.simpoole.propagatedBuildInputs ++ hspkgs.simpoole.buildInputs
+          ))
+          cabal-install
+          haskell-language-server
+          hlint
+          stylish-haskell
+          nixpkgs-fmt
+        ];
+      };
+    }
+  );
+}