build: add flake.nix

run `nix develop` to get a development environment

flake.nix

@@ -0,0 +1,68 @@
+{
+  description = "My haskell library";
+
+  nixConfig = {
+    extra-substituters = [
+      "https://haskell-language-server.cachix.org"
+    ];
+    extra-trusted-public-keys = [
+      "haskell-language-server.cachix.org-1:juFfHrwkOxqIOZShtC4YC1uT1bBcq2RSvC7OMKx0Nz8="
+    ];
+  };
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+
+    flake-utils.url = "github:numtide/flake-utils";
+
+    hls.url = "github:haskell/haskell-language-server/854d2c5a1aa3095c93e3f38ef3731be89b4ab09a";
+
+    flake-compat = {
+      url = "github:edolstra/flake-compat";
+      flake = false;
+    };
+  };
+
+  outputs = { self, nixpkgs, flake-utils, hls, ... }:
+    flake-utils.lib.eachSystem ["x86_64-linux"] (system: let
+
+      compilerVersion = "8107";
+
+      haskellOverlay = hnew: hold: with pkgs.haskell.lib; {
+      };
+
+      pkgs = import nixpkgs {
+          inherit system;
+          config = { allowUnfree = false; allowBroken = true;};
+        };
+
+      hsPkgs = pkgs.haskell.packages."ghc${compilerVersion}";
+
+      # modifier used in haskellPackages.developPackage
+      myModifier = drv:
+        pkgs.haskell.lib.addBuildTools drv (with hsPkgs; [
+          cabal-install
+          hls.packages.${system}."haskell-language-server-${compilerVersion}"
+          pkgs.openssl.dev
+          pkgs.zlib
+        ]);
+
+      mkPackage = 
+          hsPkgs.developPackage {
+            root =  pkgs.lib.cleanSource ./. ;
+            name =  "cabal2nix";
+            returnShellEnv = false;
+            withHoogle = true;
+            overrides = haskellOverlay;
+            modifier = myModifier;
+          };
+
+    in {
+      packages = {
+        cabal2nix = self.packages.${system}.cabal2nix-8107;
+        cabal2nix-8107 = mkPackage;
+      };
+
+      defaultPackage = self.packages.${system}.cabal2nix;
+  });
+}