diff --git a/snack-lib/default.nix b/snack-lib/default.nix
index fe7ec86..fc20899 100644
--- a/snack-lib/default.nix
+++ b/snack-lib/default.nix
@@ -40,7 +40,7 @@ let
   libraryModSpecs = pkgSpec:
     let
       moduleSpecFold' = modSpecFoldFromPackageSpec pkgSpec;
-      modNames = listModulesInDir pkgSpec.packageBase;
+      modNames = pkgs.lib.concatMap listModulesInDir pkgSpec.packageSourceDirs;
       fld = moduleSpecFold' modSpecs';
       modSpecs' = foldDAG fld modNames;
       modSpecs = builtins.attrValues modSpecs';
diff --git a/snack-lib/hpack.nix b/snack-lib/hpack.nix
index fa62816..9c1811f 100644
--- a/snack-lib/hpack.nix
+++ b/snack-lib/hpack.nix
@@ -37,7 +37,13 @@ in
         packageLib = withAttr package "library" null (component:
             { src =
                 let base = builtins.dirOf packageYaml;
-                in builtins.toPath "${builtins.toString base}/${component.source-dirs}";
+                in
+                  if builtins.isList component.source-dirs
+                  then builtins.map (sourceDir:
+                    builtins.toPath "${builtins.toString base}/${sourceDir}"
+                    ) component.source-dirs
+                  else
+                    builtins.toPath "${builtins.toString base}/${component.source-dirs}";
               dependencies = topDeps ++ mkDeps component;
               extensions = topExtensions ++ (optAttr component "extensions" []);
             }
@@ -55,9 +61,14 @@ in
           in
             { main = fileToModule component.main;
               src =
-                let
-                  base = builtins.dirOf packageYaml;
-                in builtins.toPath "${builtins.toString base}/${component.source-dirs}";
+                let base = builtins.dirOf packageYaml;
+                in
+                  if builtins.isList component.source-dirs
+                  then builtins.map (sourceDir:
+                    builtins.toPath "${builtins.toString base}/${sourceDir}"
+                    ) component.source-dirs
+                  else
+                    builtins.toPath "${builtins.toString base}/${component.source-dirs}";
               dependencies = topDeps ++ dropVersionBounds depOrPack.wrong;
               extensions = topExtensions ++ (optAttr component "extensions" []);
             packages = map (_: packageLib) depOrPack.right;
diff --git a/snack-lib/module-spec.nix b/snack-lib/module-spec.nix
index aada6c5..b12b543 100644
--- a/snack-lib/module-spec.nix
+++ b/snack-lib/module-spec.nix
@@ -100,8 +100,8 @@ rec {
   modSpecFoldFromPackageSpec = pkgSpec:
       let
         baseByModuleName = modName:
-          let res = pkgSpecByModuleName pkgSpec null modName;
-          in if res == null then null else res.packageBase;
+          let res = pkgSpecAndBaseByModuleName pkgSpec modName;
+          in if res == null then null else res.base;
         depsByModuleName = modName:
           (pkgSpecByModuleName
             pkgSpec
diff --git a/snack-lib/package-spec.nix b/snack-lib/package-spec.nix
index 22f6d89..adb269f 100644
--- a/snack-lib/package-spec.nix
+++ b/snack-lib/package-spec.nix
@@ -18,7 +18,10 @@ rec {
     , packages ? []
     }:
     { packageMain = main;
-      packageBase = src;
+      packageSourceDirs =
+        if builtins.isList src
+        then src
+        else [src];
       packageGhcOpts = ghcOpts;
       packageExtensions = extensions;
       packageDependencies = mkPerModuleAttr dependencies;
@@ -45,14 +48,25 @@ rec {
   # 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
+  pkgSpecAndBaseByModuleName = topPkgSpec: modName:
+    let
+      foo = pkgSpec:
+        lib.findFirst
+          (base: lib.lists.elem modName (listModulesInDir base))
+          null
+          pkgSpec.packageSourceDirs;
+      bar = lib.concatMap
         (pkgSpec:
-          lib.lists.elem
-            modName
-            (listModulesInDir pkgSpec.packageBase)
-        )
-        def
-        (flattenPackages topPkgSpec)
-    );
+          let base = foo pkgSpec;
+          in if base == null then [] else [ { inherit pkgSpec base; } ])
+        (flattenPackages topPkgSpec);
+    in if lib.length bar <= 0 then null else
+       if lib.length bar == 1 then lib.head bar
+       else abort
+        "Refusing to return base, module name was found more than once: ${modName}";
+
+  pkgSpecByModuleName = topPkgSpec: def: modName:
+    let
+      res = pkgSpecAndBaseByModuleName topPkgSpec modName;
+    in if res == null then def else res.pkgSpec;
 }