Put v1 compatibility back and fix warning (#714)

* Revert "Remove backwards compatibility with V1 code (#710)"
* Fix issue with warning not being displayed

Author: hamishmack

default.nix

@@ -1,33 +1,56 @@
-{ checkMaterialization ? false  # Allows us to easily switch on materialization checking
-, defaultCompilerNixName ? null # Quick way to override the default compiler e.g. "ghc883"
-, system ? builtins.currentSystem
-, sourcesOverride ? {}
-, ... }@args: rec {
-  sources = (import ./nix/sources.nix) // sourcesOverride;
+let haskellNix = {
+      checkMaterialization ? false,  # Allows us to easily switch on materialization checking
+      defaultCompilerNixName ? null, # Quick way to override the default compiler e.g. "ghc883"
+      system ? builtins.currentSystem,
+      sourcesOverride ? {},
+      ... }@args: rec {
+    sources = (import ./nix/sources.nix) // sourcesOverride;
 
-  config   = import ./config.nix;
-  overlays = [ allOverlays.combined ] ++ (
-    if checkMaterialization == true
-      then [(
-        final: prev: {
-          haskell-nix = prev.haskell-nix // {
-            checkMaterialization = true;
-          };
-        }
-      )]
-      else []
-  ) ++ (
-    if defaultCompilerNixName != null
-      then [(
-        final: prev: {
-          haskell-nix = prev.haskell-nix // {
-            inherit defaultCompilerNixName;
-          };
-        }
-      )]
-      else []
-  );
-  allOverlays = import ./overlays args;
-  nixpkgsArgs = { inherit config overlays system; };
-  pkgs = import sources.nixpkgs-default nixpkgsArgs;
+    config   = import ./config.nix;
+    overlays = [ allOverlays.combined ] ++ (
+      if checkMaterialization == true
+        then [(
+          final: prev: {
+            haskell-nix = prev.haskell-nix // {
+              checkMaterialization = true;
+            };
+          }
+        )]
+        else []
+    ) ++ (
+      if defaultCompilerNixName != null
+        then [(
+          final: prev: {
+            haskell-nix = prev.haskell-nix // {
+              inherit defaultCompilerNixName;
+            };
+          }
+        )]
+        else []
+    );
+    allOverlays = import ./overlays args;
+    nixpkgsArgs = { inherit config overlays system; };
+    pkgs = import sources.nixpkgs-default nixpkgsArgs;
+  };
+
+  v1DeprecationMessage = "Version 1 is deprecated: use version 2 (nixpkgs arguments are available as the `nixpkgsArgs` attribute of version 2)";
+  haskellNixV1 = (haskellNix {}).nixpkgsArgs // {
+    overlays = builtins.trace v1DeprecationMessage (haskellNix {}).nixpkgsArgs.overlays;
+  };
+  haskellNixV2 = haskellNix;
+
+# If no arguments, then you get V1
+in haskellNixV1 // {
+  __functor = _: {
+      version ? 2,
+      checkMaterialization ? false,  # Allows us to easily switch on materialization checking
+      defaultCompilerNixName ? null, # Quick way to override the default compiler e.g. "ghc883"
+      system ? builtins.currentSystem,
+      sourcesOverride ? {},
+      ... }@args:
+    if version == 1
+    then haskellNixV1
+    else if version == 2
+    then haskellNixV2 args
+    else builtins.throw ("haskell.nix: unknown version: " + (builtins.toString version));
 }