diff --git a/myShell.nix b/myShell.nix
new file mode 100644
index 0000000..4d3be4f
--- /dev/null
+++ b/myShell.nix
@@ -0,0 +1,57 @@
+let
+
+myHaskellPackageOverlay = self: super: {
+
+  myHaskellPackages = super.haskellPackages.override {
+    overrides = hself: hsuper: rec {
+
+      cas-store = self.haskell.lib.overrideCabal self.haskellPackages.cas-store (old: { libraryHaskellDepends = old.libraryHaskellDepends ++ [ self.haskellPackages.kqueue ]; });
+
+      funflow = super.haskell.lib.dontCheck (hself.callHackage "funflow" "1.5.0" {});
+
+      funflow-nix = super.haskell.lib.dontCheck hsuper.funflow-nix;
+
+
+    };
+  };
+};
+
+in
+
+{ nixpkgs ? import <nixpkgs> { config.allowBroken = true;
+                               overlays = [ myHaskellPackageOverlay ];
+                             },
+  compiler ? "default",
+  doBenchmark ? false }:
+
+let
+
+  inherit (nixpkgs) pkgs;
+
+  f = { mkDerivation, base, containers, data-default, funflow
+      , funflow-nix, Glob, lib, path, path-io, time, unix
+      }:
+      mkDerivation {
+        pname = "map-scraper";
+        version = "0.1.0.0";
+        src = ./.;
+        isLibrary = false;
+        isExecutable = true;
+        executableHaskellDepends = [
+          base containers data-default funflow funflow-nix Glob path path-io
+          time unix
+        ];
+        license = lib.licenses.bsd3;
+      };
+
+  haskellPackages = if compiler == "default"
+                       then pkgs.haskellPackages
+                       else pkgs.haskell.packages.${compiler};
+
+  variant = if doBenchmark then pkgs.haskell.lib.doBenchmark else pkgs.lib.id;
+
+  drv = variant (pkgs.myHaskellPackages.callPackage f {});
+
+in
+
+  if pkgs.lib.inNixShell then drv.env else drv
diff --git a/shell.nix b/shell.nix
index ff5d563..a2fd30e 100644
--- a/shell.nix
+++ b/shell.nix
@@ -1,4 +1,4 @@
 let
   np = import <nixpkgs> {};
 in
-  np.mkShell { buildInputs = [np.haskell.packages.ghc822.ghc]; }
+  np.mkShell { buildInputs = [np.haskell.packages.ghc8104.ghc]; }
diff --git a/src/Main.hs b/src/Main.hs
index 5e3e617..fb5be08 100644
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -3,6 +3,8 @@
 {-# LANGUAGE QuasiQuotes #-}
 {-# LANGUAGE TupleSections #-}
 
+{-# OPTIONS_GHC -Wall -Wno-type-defaults #-}
+
 module Main where
 
 import Control.Arrow
@@ -57,7 +59,7 @@ partitionA f = proc xs ->
         That d -> ((second (partitionA f)) >>> arr (\(d, (cs, ds)) -> (cs, d:ds))) -< (d, ys)
         These c d -> (second (partitionA f)) >>> arr (\((c, d), (cs, ds)) -> (c:cs, d:ds)) -< ((c, d), ys)
 
-boolChoice :: ArrowFlow eff ex a => a CS.Item (These CS.Item (Content Dir))
+boolChoice :: (ArrowFlow eff ex a, ArrowChoice a) => a CS.Item (These CS.Item (Content Dir))
 boolChoice = proc dir -> do
   str <- readString_ -< dir
   case str of
@@ -250,8 +252,8 @@ nixScriptX impure std script scripts params = proc (scriptDir, a) -> do
         { _etCommand = "perl"
         , _etParams = contentParam (s ^</> script) : params args
         , _etWriteToStdOut = std
-        , _etEnv = [("NIX_PATH",  textParam "nixpkgs="
-                               <> textParam nixPackages )] }) -< (env, a)
+        , _etEnv = EnvExplicit [("NIX_PATH",  textParam "nixpkgs="
+                                  <> textParam nixPackages )] }) -< (env, a)
   where
     props = def { ep_impure = impure }
     absScripts sd = map (sd ^</>) (script : scripts)
@@ -281,7 +283,7 @@ mergeDirs' = proc inDirs -> do
 rmOut :: ArrowFlow eff ex arr => arr (Content Dir) (Content Dir)
 rmOut = proc dir -> do mergeFiles <<< globDir -< (dir, "*.pickle")
 
-splitDir :: ArrowFlow eff ex arr => arr (Content Dir) ([Content File])
+splitDir :: (ArrowFlow eff ex arr, ArrowChoice arr) => arr (Content Dir) ([Content File])
 splitDir = proc dir -> do
   (_, fs) <- listDirContents -< dir
   mapA reifyFile -< fs