Added flake support

flake.nix

@@ -0,0 +1,52 @@
+{
+  description = "Opt-in Stack Flake";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+
+        hPkgs =
+          pkgs.haskell.packages."ghc927"; # need to match Stackage LTS version from stack.yaml resolver
+
+        myDevTools = [
+          hPkgs.ghc # GHC compiler in the desired version (will be available on PATH)
+          stack-wrapped
+          pkgs.zlib # External C library needed by some Haskell packages
+          pkgs.secp256k1
+          pkgs.libff
+        ];
+
+        stack-wrapped = pkgs.symlinkJoin {
+          name = "stack"; # will be available as the usual `stack` in terminal
+          paths = [ pkgs.stack ];
+          buildInputs = [ pkgs.makeWrapper ];
+          postBuild = ''
+            wrapProgram $out/bin/stack \
+              --add-flags "\
+                --no-nix \
+                --system-ghc \
+                --no-install-ghc \
+              "
+          '';
+        };
+      in {
+        devShells.default = pkgs.mkShell {
+          buildInputs = myDevTools;
+
+          # Make external Nix c libraries like zlib known to GHC, like
+          # pkgs.haskell.lib.buildStackProject does
+          LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath myDevTools;
+
+          shellHook = ''
+            if test -f "REPO_UNINITIALIZED.sh"; then
+              chmod +x REPO_UNINITIALIZED.sh
+              ./REPO_UNINITIALIZED.sh
+              rm REPO_UNINITIALIZED.sh
+            fi
+          '';
+        };
+      });
+}

stack.yaml

@@ -75,7 +75,7 @@ allow-newer: true
 #
 # Override the architecture used by stack, especially useful on Windows
 # Change to x86_64 for Intel and aarch64 for ARM/apple silicon
-arch: x86_64
+arch: aarch64
 #
 # Extra directories used by stack for building
 # extra-include-dirs: [/path/to/dir]
@@ -86,7 +86,3 @@ arch: x86_64
 #
 ghc-options:
   "$locals": -fwarn-incomplete-patterns
-
-nix:
-  enable: true
-  packages: [libff, secp256k1, zlib]