Add a Nix flake

Add a basic nix flake that simply exports several packages from the
toplevel default.nix

flake.nix

@@ -0,0 +1,53 @@
+{
+  description = "Flake for Plutus";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs?rev=7d71001b796340b219d1bfa8552c81995017544a";
+    haskell-nix.url = "github:input-output-hk/haskell.nix?rev=87084d65a476cc826a0e8c5d281d494254f5bc7a";
+    flake-utils.url = "github:numtide/flake-utils?rev=b543720b25df6ffdfcf9227afafc5b8c1fabfae8";
+  };
+
+  outputs = { self, nixpkgs, haskell-nix, flake-utils, ... }:
+    (flake-utils.lib.eachSystem [ "x86_64-linux" ] (system:
+      let
+        #
+        # Obtain all niv sources extending it with the 'nixpkgs' flake input
+        #
+        sources =
+          let
+            sourcesInfo = builtins.fromJSON (builtins.readFile ./nix/sources.json);
+            fetch = sourceInfo: builtins.fetchTarball { inherit (sourceInfo) url sha256; };
+          in
+          builtins.mapAttrs (name: info: fetch info) sourcesInfo // { inherit nixpkgs; };
+
+        #
+        # all packages from nix/default.nix
+        #
+        plutusPackages = import ./nix {
+          inherit system sources;
+          haskellNixOverlays = [ haskell-nix.overlay ];
+        };
+
+        #
+        # all packages from ./default.nix
+        #
+        topLevel = import ./. {
+          inherit system;
+          packages = plutusPackages;
+        };
+
+        inherit (plutusPackages) pkgs plutus ownOverlays;
+        inherit (plutus) haskell iohkNix;
+        inherit (plutus.lib) buildPursPackage buildNodeModules filterNpm gitignore-nix;
+      in
+      rec {
+        packages = rec {
+          marlowe-playground-client = topLevel.marlowe-playground.client;
+          marlowe-playground-server = topLevel.marlowe-playground.server;
+          plutus-playground-client = topLevel.plutus-playground.client;
+          plutus-playground-server = topLevel.plutus-playground.server;
+          marlowe-website = topLevel.marlowe-web;
+          web-ghc-server = plutus.haskell.project.hsPkgs.web-ghc.components.exes.web-ghc-server;
+        };
+      }));
+}

nix/default.nix

@@ -3,30 +3,31 @@
 , config ? { }
 , overlays ? [ ]
 , sourcesOverride ? { }
-, checkMaterialization ? false
-, enableHaskellProfiling ? false
-}:
-let
-  sources = import ./sources.nix { inherit pkgs; }
-    // sourcesOverride;
-  haskellNix = import sources."haskell.nix" {
+, sources ? import ./sources.nix { } // sourcesOverride
+, haskellNix ? import sources."haskell.nix" {
     sourcesOverride = {
       hackage = sources."hackage.nix";
       stackage = sources."stackage.nix";
     };
-  };
-
-  extraOverlays =
-    # Haskell.nix (https://github.com/input-output-hk/haskell.nix)
-    haskellNix.overlays
-    # our own overlays:
-    ++ [
+  }
+  # haskell-nix has to be used differently in flakes/no-flakes scenarios:
+  # - When imported from flakes, 'haskellNix.overlay' needs to be passed here.
+  # - When imported from default.nix without flakes, default to haskellNix.overlays
+, haskellNixOverlays ? haskellNix.overlays
+, checkMaterialization ? false
+, enableHaskellProfiling ? false
+}:
+let
+  ownOverlays =
+    [
       # Modifications to derivations from nixpkgs
       (import ./overlays/nixpkgs-overrides.nix)
       # fix r-modules
       (import ./overlays/r.nix)
     ];
 
+  extraOverlays = haskellNixOverlays ++ ownOverlays;
+
   pkgs = import sources.nixpkgs {
     inherit system crossSystem;
     overlays = extraOverlays ++ overlays;
@@ -37,5 +38,5 @@ let
 
 in
 {
-  inherit pkgs plutus sources;
+  inherit pkgs plutus sources ownOverlays;
 }