diff --git a/bin/snack b/bin/snack
index 621437e..38cebad 100755
--- a/bin/snack
+++ b/bin/snack
@@ -3,8 +3,10 @@ set -euo pipefail
 
 ## Defaults
 
+NIXPKGS=
 NIX_BUILD=nix-build
-SNACK_NIX=snack.nix
+SNACK_NIX="./snack.nix"
+WRAPPER_NIX=
 COMMAND=
 
 ## Functions
@@ -21,6 +23,12 @@ Snack is a Haskell build tool
 
 Options:
   -f | --snack-nix <PATH>: sets the path ot the "snack.nix" file. Default: "./snack.nix"
+  -w | --wrapper-nix <PATH>: sets the path ot the nix wrapper file. This file
+        should take at least one argument, "snackNix", which is the path to the
+        "snack.nix".
+  -n | --nixpkgs <PATH>: use the path to import nixpkgs. The expression should
+        take no arguments and evaluate to a set containing at least
+        "snack-lib".
   -h | --help: Shows this help
 
 Commands:
@@ -41,6 +49,16 @@ while [[ $# -gt 0 ]]; do
       shift
       shift
       ;;
+    -w | --wrapper-nix)
+      WRAPPER_NIX="$2"
+      shift
+      shift
+      ;;
+    -n | --nixpkgs)
+      NIXPKGS="$2"
+      shift
+      shift
+      ;;
     -h | --help)
       show_usage
       exit 0
@@ -62,16 +80,31 @@ if [[ -z "$COMMAND" ]]; then
   exit 1
 fi
 
+if [[ -z "$WRAPPER_NIX" ]]; then
+  log_error "missing <wrapper>\n"
+  show_usage
+  exit 1
+fi
+
+call_snack() {
+  "$NIX_BUILD" \
+    --no-out-link \
+    -A $1 \
+    "$WRAPPER_NIX" \
+    --arg snackNix "$SNACK_NIX" \
+    --arg nixpkgs "$NIXPKGS"
+}
+
 case $COMMAND in
   build)
-    "$NIX_BUILD" --no-out-link --show-trace -A build "$SNACK_NIX"
+    call_snack build
     ;;
   ghci)
-    res=$("$NIX_BUILD" --no-out-link -A ghci "$SNACK_NIX")
+    res=$(call_snack ghci)
     "$res"
     ;;
   run)
-    res=$("$NIX_BUILD" --no-out-link -A build "$SNACK_NIX")
+    res=$(call_snack build)
     "$res/out"
     ;;
 esac
diff --git a/nix/overlay.nix b/nix/overlay.nix
index 98c74d9..ac3ebd2 100644
--- a/nix/overlay.nix
+++ b/nix/overlay.nix
@@ -3,7 +3,7 @@ _: pkgs: {
   snack-exe = pkgs.writeScriptBin
     "snack"
     (builtins.replaceStrings
-      ["NIX_BUILD=nix-build"]
-      ["NIX_BUILD=${pkgs.nix}/bin/nix-build"]
+      ["NIX_BUILD=nix-build"  "WRAPPER_NIX="]
+      ["NIX_BUILD=${pkgs.nix}/bin/nix-build"  "WRAPPER_NIX=${../snack-lib/wrapper.nix}"]
       (builtins.readFile ../bin/snack));
 }
diff --git a/script/test b/script/test
index fe7787f..658d340 100755
--- a/script/test
+++ b/script/test
@@ -33,6 +33,8 @@ END_HEREDOC
 
 export -f capture_io
 
+export SNACK="snack -n $(readlink -f ./nix)"
+
 fail() {
   echo "ERROR: $*"
   exit 1
diff --git a/snack-lib/default.nix b/snack-lib/default.nix
index c3d1b22..7f450eb 100644
--- a/snack-lib/default.nix
+++ b/snack-lib/default.nix
@@ -204,58 +204,50 @@ let
   executable = pkgDescr:
       let
         topPkgSpec = mkPackageSpec pkgDescr;
-        pkgs = flattenPackages topPkgSpec;
         baseByModuleName = modName:
-          (pkgSpecByModuleName pkgs topPkgSpec modName).packageBase;
+          let res = pkgSpecByModuleName topPkgSpec null modName;
+          in if res == null then null else res.packageBase;
 
         depsByModuleName = modName:
-          (pkgSpecByModuleName pkgs (abort "should not happen") modName).packageDependencies;
+          (pkgSpecByModuleName
+            topPkgSpec
+            (abort "asking dependencies for external module: ${modName}")
+            modName).packageDependencies;
         ghcOptsByModuleName = modName:
-          (pkgSpecByModuleName pkgs (abort "should not happen") modName).packageGhcOpts;
+          (pkgSpecByModuleName
+            topPkgSpec
+            (abort "asking ghc options for external module: ${modName}")
+            modName).packageGhcOpts;
 
         ghcWith = deps: haskellPackages.ghcWithPackages
           (ps: map (p: ps.${p}) deps);
-        ghcOpts = topPkgSpec.packageGhcOpts;
         base = topPkgSpec.packageBase;
         extraFiles =  topPkgSpec.packageExtraFiles;
         extraDirs = topPkgSpec.packageExtraDirectories;
         mainModName = topPkgSpec.packageMain;
+        topModuleSpec =
+          makeModuleSpecRec
+            baseByModuleName
+            extraFiles
+            extraDirs
+            depsByModuleName
+            ghcOptsByModuleName
+            mainModName;
       in
     {
       build =
         linkModuleObjects
           ghcWith
-          (makeModuleSpecRec
-            baseByModuleName
-            extraFiles
-            extraDirs
-            depsByModuleName
-            ghcOptsByModuleName
-            mainModName);
+          topModuleSpec;
       ghci =
         ghciExecutable
           ghcWith
-          ghcOpts
-          (makeModuleSpecRec
-            baseByModuleName
-            extraFiles
-            extraDirs
-            depsByModuleName
-            ghcOptsByModuleName
-            mainModName);
+          (allTransitiveGhcOpts topPkgSpec)
+          topModuleSpec;
     };
-  library =
-    { src
-    , dependencies ? [] # TODO: handle this
-    }:
-        {
-          inherit src dependencies;
-          # TODO: add build for libraries
-        };
 in
   {
     inherit
     executable
-    library
     ;
   }
diff --git a/snack-lib/module-spec.nix b/snack-lib/module-spec.nix
index 27ff2f1..773eab1 100644
--- a/snack-lib/module-spec.nix
+++ b/snack-lib/module-spec.nix
@@ -42,7 +42,7 @@ rec {
         makeModuleSpec
           modName
           (map (f false)
-            (listModuleImports baseByModuleName modName)
+            (lib.lists.filter (mn: baseByModuleName mn != null) (listModuleImports baseByModuleName modName))
           )
           isMain
           (filesByModuleName modName)
diff --git a/snack-lib/modules.nix b/snack-lib/modules.nix
index 470ae3e..727427b 100644
--- a/snack-lib/modules.nix
+++ b/snack-lib/modules.nix
@@ -29,22 +29,17 @@ rec {
   singleOutModulePath = base: mod:
     "${singleOut base (moduleToFile mod)}/${moduleToFile mod}";
 
-  # Generate a list of haskell module names needed by the haskell file,
-  # excluding modules that are not present in this project/base
+  # Generate a list of haskell module names needed by the haskell file
   listModuleImports = baseByModuleName: modName:
-    lib.filter
-      (doesModuleExist baseByModuleName)
-      (builtins.fromJSON
-        (builtins.readFile (listAllModuleImportsJSON (baseByModuleName modName) modName))
-      );
+    builtins.fromJSON
+     (builtins.readFile (listAllModuleImportsJSON (baseByModuleName modName) modName))
+    ;
 
   listModulesInDir = dir: map fileToModule (listFilesInDir dir);
 
-
   doesModuleExist = baseByModuleName: modName:
     doesFileExist (baseByModuleName modName) (moduleToFile modName);
 
-
   # Lists all module dependencies, not limited to modules existing in this
   # project
   listAllModuleImportsJSON = base: modName:
diff --git a/snack-lib/package-spec.nix b/snack-lib/package-spec.nix
index e4329eb..c0369f7 100644
--- a/snack-lib/package-spec.nix
+++ b/snack-lib/package-spec.nix
@@ -43,10 +43,17 @@ rec {
     (pkgSpec: pkgSpec.packageDependencies)
     (flattenPackages topPkgSpec);
 
+  # TODO: nub
+  allTransitiveGhcOpts = topPkgSpec:
+    lib.lists.concatMap
+    (pkgSpec: pkgSpec.packageGhcOpts)
+    (flattenPackages topPkgSpec);
 
-  # Returns the first package spec that contains a module with given name. If
-  # none is found, returns the supplied default value.
-  pkgSpecByModuleName = pkgs: def: modName:
+
+  # Traverses all transitive packages and returns the first package spec that
+  # contains a module with given name. If none is found, returns the supplied
+  # default value.
+  pkgSpecByModuleName = topPkgSpec: def: modName:
     ( lib.findFirst
         (pkgSpec:
           lib.lists.elem
@@ -54,6 +61,6 @@ rec {
             (listModulesInDir pkgSpec.packageBase)
         )
         def
-        pkgs
+        (flattenPackages topPkgSpec)
     );
 }
diff --git a/snack-lib/wrapper.nix b/snack-lib/wrapper.nix
new file mode 100644
index 0000000..3dba784
--- /dev/null
+++ b/snack-lib/wrapper.nix
@@ -0,0 +1,14 @@
+{ snackNix
+, nixpkgs ? null
+}:
+  let
+    pkgs =
+      if nixpkgs == null
+      then import <nixpkgs> {}
+      else import nixpkgs {};
+    snack = pkgs.snack-lib;
+  in
+{
+  build = (snack.executable (import snackNix)).build;
+  ghci = (snack.executable (import snackNix)).ghci;
+}
diff --git a/tests/library/snack.nix b/tests/library/snack.nix
index 6401b9c..0e458cc 100644
--- a/tests/library/snack.nix
+++ b/tests/library/snack.nix
@@ -1,13 +1,10 @@
 let
-  pkgs = import ../../nix {};
-  snack = pkgs.snack-lib;
-  my-lib = snack.library
+  my-lib =
     { src = ./src;
       dependencies = [ "conduit" ];
     };
 in
-  snack.executable
-    { main = "Foo";
-      src = ./app;
-      dependencies = [ my-lib "conduit" ];
-    }
+  { main = "Foo";
+    src = ./app;
+    dependencies = [ my-lib "conduit" ];
+  }
diff --git a/tests/library/test b/tests/library/test
index 5dd0de1..71e3896 100755
--- a/tests/library/test
+++ b/tests/library/test
@@ -3,12 +3,12 @@
 
 set -euo pipefail
 
-snack build
-snack run | diff golden -
+$SNACK build
+$SNACK run | diff golden -
 
 TMP_FILE=$(mktemp)
 
-capture_io "$TMP_FILE" main | snack ghci
+capture_io "$TMP_FILE" main | $SNACK ghci
 
 diff golden $TMP_FILE
 rm $TMP_FILE
diff --git a/tests/packages/snack.nix b/tests/packages/snack.nix
index 30a22f9..f74c554 100644
--- a/tests/packages/snack.nix
+++ b/tests/packages/snack.nix
@@ -1,9 +1,4 @@
-let
-  pkgs = import ../../nix {};
-  snack = pkgs.snack-lib;
-in
-  snack.executable
-    { main = "Foo";
-      src = ./src;
-      dependencies = ["conduit"];
-    }
+{ main = "Foo";
+  src = ./src;
+  dependencies = ["conduit"];
+}
diff --git a/tests/packages/test b/tests/packages/test
index 5dd0de1..71e3896 100755
--- a/tests/packages/test
+++ b/tests/packages/test
@@ -3,12 +3,12 @@
 
 set -euo pipefail
 
-snack build
-snack run | diff golden -
+$SNACK build
+$SNACK run | diff golden -
 
 TMP_FILE=$(mktemp)
 
-capture_io "$TMP_FILE" main | snack ghci
+capture_io "$TMP_FILE" main | $SNACK ghci
 
 diff golden $TMP_FILE
 rm $TMP_FILE
diff --git a/tests/template-haskell-2/code/snack.nix b/tests/template-haskell-2/code/snack.nix
index 508b905..292fe3a 100644
--- a/tests/template-haskell-2/code/snack.nix
+++ b/tests/template-haskell-2/code/snack.nix
@@ -1,11 +1,6 @@
-let
-  pkgs = import ../../../nix {};
-  snack = pkgs.snack-lib;
-in
-  snack.executable
-    { main = "Main";
-      src = ./.;
-      dependencies = ["file-embed"];
-      extra-directories =
-        (modName: if modName == "Main" then [ ../. ] else []);
-    }
+{ main = "Main";
+  src = ./.;
+  dependencies = ["file-embed"];
+  extra-directories =
+    (modName: if modName == "Main" then [ ../. ] else []);
+}
diff --git a/tests/template-haskell-2/test b/tests/template-haskell-2/test
index 81788c3..7e39d4e 100755
--- a/tests/template-haskell-2/test
+++ b/tests/template-haskell-2/test
@@ -3,12 +3,12 @@
 
 set -euo pipefail
 
-snack build -f code/snack.nix
-snack run -f code/snack.nix | diff golden -
+$SNACK build -f code/snack.nix
+$SNACK run -f code/snack.nix | diff golden -
 
 TMP_FILE=$(mktemp)
 
-capture_io "$TMP_FILE" main | snack -f code/snack.nix ghci
+capture_io "$TMP_FILE" main | $SNACK -f code/snack.nix ghci
 
 diff golden $TMP_FILE
 rm $TMP_FILE
diff --git a/tests/template-haskell-3/snack.nix b/tests/template-haskell-3/snack.nix
index b98b5bf..a0c45d1 100644
--- a/tests/template-haskell-3/snack.nix
+++ b/tests/template-haskell-3/snack.nix
@@ -1,11 +1,6 @@
-let
-  pkgs = import ../../nix {};
-  snack = pkgs.snack-lib;
-in
-  snack.executable
-    { main = "Main";
-      src = ./.;
-      dependencies = ["file-embed"];
-      extra-files =
-        (modName: if modName == "Main" then [ "assets/foo.txt" ] else []);
-    }
+{ main = "Main";
+  src = ./.;
+  dependencies = ["file-embed"];
+  extra-files =
+    (modName: if modName == "Main" then [ "assets/foo.txt" ] else []);
+}
diff --git a/tests/template-haskell-3/test b/tests/template-haskell-3/test
index 5dd0de1..71e3896 100755
--- a/tests/template-haskell-3/test
+++ b/tests/template-haskell-3/test
@@ -3,12 +3,12 @@
 
 set -euo pipefail
 
-snack build
-snack run | diff golden -
+$SNACK build
+$SNACK run | diff golden -
 
 TMP_FILE=$(mktemp)
 
-capture_io "$TMP_FILE" main | snack ghci
+capture_io "$TMP_FILE" main | $SNACK ghci
 
 diff golden $TMP_FILE
 rm $TMP_FILE
diff --git a/tests/template-haskell-4/snack.nix b/tests/template-haskell-4/snack.nix
index 3c5c305..3011fe3 100644
--- a/tests/template-haskell-4/snack.nix
+++ b/tests/template-haskell-4/snack.nix
@@ -1,11 +1,6 @@
-let
-  pkgs = import ../../nix {};
-  snack = pkgs.snack-lib;
-in
-  snack.executable
-    { main = "Main";
-      src = ./src;
-      dependencies = ["file-embed"];
-      extra-directories =
-        (modName: if modName == "Main" then [ ./assets ] else []);
-    }
+{ main = "Main";
+  src = ./src;
+  dependencies = ["file-embed"];
+  extra-directories =
+    (modName: if modName == "Main" then [ ./assets ] else []);
+}
diff --git a/tests/template-haskell-4/test b/tests/template-haskell-4/test
index 5dd0de1..71e3896 100755
--- a/tests/template-haskell-4/test
+++ b/tests/template-haskell-4/test
@@ -3,12 +3,12 @@
 
 set -euo pipefail
 
-snack build
-snack run | diff golden -
+$SNACK build
+$SNACK run | diff golden -
 
 TMP_FILE=$(mktemp)
 
-capture_io "$TMP_FILE" main | snack ghci
+capture_io "$TMP_FILE" main | $SNACK ghci
 
 diff golden $TMP_FILE
 rm $TMP_FILE
diff --git a/tests/template-haskell/snack.nix b/tests/template-haskell/snack.nix
index 36418dc..2c9dd3d 100644
--- a/tests/template-haskell/snack.nix
+++ b/tests/template-haskell/snack.nix
@@ -1,11 +1,6 @@
-let
-  pkgs = import ../../nix {};
-  snack = pkgs.snack-lib;
-in
-  snack.executable
-    { main = "Main";
-      src = ./.;
-      dependencies = ["file-embed"];
-      extra-files =
-        (modName: if modName == "Main" then [ "foo.txt" ] else []);
-    }
+{ main = "Main";
+  src = ./.;
+  dependencies = ["file-embed"];
+  extra-files =
+    (modName: if modName == "Main" then [ "foo.txt" ] else []);
+}
diff --git a/tests/template-haskell/test b/tests/template-haskell/test
index 5dd0de1..71e3896 100755
--- a/tests/template-haskell/test
+++ b/tests/template-haskell/test
@@ -3,12 +3,12 @@
 
 set -euo pipefail
 
-snack build
-snack run | diff golden -
+$SNACK build
+$SNACK run | diff golden -
 
 TMP_FILE=$(mktemp)
 
-capture_io "$TMP_FILE" main | snack ghci
+capture_io "$TMP_FILE" main | $SNACK ghci
 
 diff golden $TMP_FILE
 rm $TMP_FILE