diff --git a/distribution-nixpkgs.cabal b/distribution-nixpkgs.cabal
--- a/distribution-nixpkgs.cabal
+++ b/distribution-nixpkgs.cabal
@@ -1,56 +1,55 @@
-name:           distribution-nixpkgs
-version:        1.1.1
-synopsis:       Types and functions to manipulate the Nixpkgs distribution
-description:    Types and functions to represent, query, and manipulate the Nixpkgs distribution.
-category:       Distribution, Nix
-homepage:       https://github.com/peti/distribution-nixpkgs#readme
-bug-reports:    https://github.com/peti/distribution-nixpkgs/issues
-maintainer:     Peter Simons <simons@cryp.to>
-license:        BSD3
-license-file:   LICENSE
-build-type:     Simple
-cabal-version:  >= 1.10
-tested-with:    GHC == 7.10.3, GHC == 8.0.2, GHC == 8.2.2, GHC == 8.4.1
+name:          distribution-nixpkgs
+version:       1.2
+synopsis:      Types and functions to manipulate the Nixpkgs distribution
+description:   Types and functions to represent, query, and manipulate the Nixpkgs distribution.
+license:       BSD3
+license-file:  LICENSE
+maintainer:    Peter Simons <simons@cryp.to>
+tested-with:   GHC == 7.10.3, GHC == 8.0.2, GHC == 8.2.2, GHC == 8.4.4, GHC == 8.6.3
+category:      Distribution, Nix
+homepage:      https://github.com/peti/distribution-nixpkgs#readme
+bug-reports:   https://github.com/peti/distribution-nixpkgs/issues
+build-type:    Simple
+cabal-version: >= 1.10
 
 source-repository head
-  type: git
+  type:     git
   location: https://github.com/peti/distribution-nixpkgs
 
 library
-  hs-source-dirs:
-      src
-  other-extensions: DeriveGeneric PackageImports RecordWildCards TemplateHaskell
-  ghc-options: -Wall
-  build-depends:
-      aeson
-    , base > 4.2 && < 5
-    , bytestring
-    , Cabal > 2.2
-    , containers
-    , deepseq >= 1.4
-    , language-nix > 2
-    , lens
-    , pretty >= 1.1.2
-    , process
-    , split
-  exposed-modules:
-      Distribution.Nixpkgs.License
-      Distribution.Nixpkgs.Meta
-      Distribution.Nixpkgs.PackageMap
-      Distribution.Nixpkgs.Hashes
-      Language.Nix.PrettyPrinting
-  default-language: Haskell2010
+  exposed-modules:    Distribution.Nixpkgs.Hashes
+                      Distribution.Nixpkgs.License
+                      Distribution.Nixpkgs.Meta
+                      Distribution.Nixpkgs.PackageMap
+                      Language.Nix.PrettyPrinting
+  hs-source-dirs:     src
+  build-depends:      base         > 4.2 && < 5
+                    , Cabal        > 2.2
+                    , aeson
+                    , bytestring
+                    , containers
+                    , deepseq      >= 1.4
+                    , language-nix > 2
+                    , lens
+                    , pretty       >= 1.1.2
+                    , process
+                    , split
+  default-language:   Haskell2010
+  default-extensions: MonadFailDesugaring
+  other-extensions:   CPP
+                      TemplateHaskell
+                      RecordWildCards
+                      DeriveGeneric
+                      PackageImports
+  ghc-options:        -Wall -Wcompat -Wincomplete-uni-patterns -Wincomplete-record-updates
+                      -Wredundant-constraints
 
 test-suite hspec
-  type: exitcode-stdio-1.0
-  main-is: hspec.hs
-  hs-source-dirs:
-      test
-  ghc-options: -Wall
-  build-depends:
-      base
-    , deepseq
-    , distribution-nixpkgs
-    , hspec
-    , lens
-  default-language: Haskell2010
+  type:               exitcode-stdio-1.0
+  main-is:            hspec.hs
+  hs-source-dirs:     test
+  build-depends:      base, deepseq, distribution-nixpkgs, hspec, lens
+  default-language:   Haskell2010
+  default-extensions: MonadFailDesugaring
+  ghc-options:        -Wall -Wcompat -Wincomplete-uni-patterns -Wincomplete-record-updates
+                      -Wredundant-constraints
diff --git a/src/Distribution/Nixpkgs/PackageMap.hs b/src/Distribution/Nixpkgs/PackageMap.hs
--- a/src/Distribution/Nixpkgs/PackageMap.hs
+++ b/src/Distribution/Nixpkgs/PackageMap.hs
@@ -3,30 +3,28 @@
   , resolve
   ) where
 
+import Control.Lens
 import qualified Data.Aeson as JSON
 import qualified Data.ByteString.Lazy as LBS
 import Data.Function
 import Data.List as List
 import Data.List.Split
-import qualified Data.Map.Strict as Map
 import Data.Map.Strict ( Map )
+import qualified Data.Map.Strict as Map
 import Data.Maybe
 import Data.Set ( Set )
 import qualified Data.Set as Set
-import Distribution.Text
 import Language.Nix
 import System.Process
-import Control.Lens
 
 type PackageMap = Map Identifier (Set Path)
 
-readNixpkgPackageMap :: FilePath -> Maybe Path -> IO PackageMap
-readNixpkgPackageMap nixpkgs attrpath = fmap identifierSet2PackageMap (readNixpkgSet nixpkgs attrpath)
+readNixpkgPackageMap :: [String] -> IO PackageMap
+readNixpkgPackageMap = fmap identifierSet2PackageMap . readNixpkgSet
 
-readNixpkgSet :: FilePath -> Maybe Path -> IO (Set String)
-readNixpkgSet nixpkgs attrpath = do
-  let extraArgs = maybe [] (\p -> ["-A", display p]) attrpath
-  (_, Just h, _, _) <- createProcess (proc "nix-env" (["-qaP", "--json", "-f", nixpkgs] ++ extraArgs))
+readNixpkgSet :: [String] -> IO (Set String)
+readNixpkgSet extraArgs = do
+  (_, Just h, _, _) <- createProcess (proc "nix-env" (["-qaP", "--json"] ++ extraArgs))
                        { std_out = CreatePipe, env = Nothing }  -- TODO: ensure that overrides don't screw up our results
   buf <- LBS.hGetContents h
   let pkgmap :: Either String (Map String JSON.Object)