diff --git a/.circleci/config.yml b/.circleci/config.yml
index 89256e4..452afd2 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -22,6 +22,7 @@ commands:
       - run:
           name: "cachix"
           command: |
+            cachix use iohk
             cachix use free-algebras
       - checkout
   cachix-push:
@@ -35,15 +36,28 @@ commands:
           name: "cachix push"
           command: cachix push free-algebras << parameters.o >>
 jobs:
-  GHC881:
+  GHC8101:
     <<: *defaults
     steps:
       - setup
       - run:
-          name: "GHC 881"
+          name: "GHC 8.10.1"
           command: |
-            nix-build release.nix -A free-category --argstr compiler ghc881 -o result-fc
-            nix-build release.nix -A examples      --argstr compiler ghc881 -o result-ex
+            nix-build -A free-category.components.all --argstr compiler ghc8101 -o result-fc
+            nix-build -A examples.components.all      --argstr compiler ghc8101 -o result-ex
+      - cachix-push:
+          o: "./result-fc"
+      - cachix-push:
+          o: "./result-ex"
+  GHC883:
+    <<: *defaults
+    steps:
+      - setup
+      - run:
+          name: "GHC 8.8.3"
+          command: |
+            nix-build -A free-category.components.all --argstr compiler ghc883 -o result-fc
+            nix-build -A examples.components.all      --argstr compiler ghc883 -o result-ex
       - cachix-push:
           o: "./result-fc"
       - cachix-push:
@@ -53,50 +67,10 @@ jobs:
     steps:
       - setup
       - run:
-          name: "GHC 865"
+          name: "GHC 8.6.5"
           command: |
-            nix-build release.nix -A free-category --argstr compiler ghc865 -o result-fc
-            nix-build release.nix -A examples      --argstr compiler ghc865 -o result-ex
-      - cachix-push:
-          o: result-fc
-      - cachix-push:
-          o: result-ex
-  GHC844:
-    <<: *defaults
-    steps:
-      - setup
-      - run:
-          name: "GHC 844"
-          command: |
-            nix-build release.nix -A free-category -o result-fc --argstr compiler ghc844
-            nix-build release.nix -A examples -o result-ex --argstr compiler ghc844
-      - cachix-push:
-          o: result-fc
-      - cachix-push:
-          o: result-ex
-  GHC822:
-    <<: *defaults
-    steps:
-      - setup
-      - run:
-          name: "GHC 822"
-          command: |
-            nix-build release.nix -A free-category -o result-fc --argstr compiler ghc822
-            nix-build release.nix -A examples -o result-ex --argstr compiler ghc822
-      - cachix-push:
-          o: result-fc
-      - cachix-push:
-          o: result-ex
-          
-  GHC802:
-    <<: *defaults
-    steps:
-      - setup
-      - run:
-          name: "GHC 802"
-          command: |
-            nix-build release.nix -A free-category --argstr compiler ghc802 -o result-fc
-            nix-build release.nix -A examples     --argstr compiler ghc802 -o result-ex
+            nix-build -A free-category.components.all --argstr compiler ghc865 -o result-fc
+            nix-build -A examples.components.all      --argstr compiler ghc865 -o result-ex
       - cachix-push:
           o: result-fc
       - cachix-push:
@@ -105,9 +79,7 @@ jobs:
 workflows:
   build:
     jobs:
-      - GHC881
+      - GHC8101
+      - GHC883
       - GHC865
-      - GHC844
-      - GHC822
-      - GHC802
 
diff --git a/cabal.project b/cabal.project
index f3f3de4..7c992a8 100644
--- a/cabal.project
+++ b/cabal.project
@@ -1,2 +1,3 @@
-packages: .
-	  ./examples
+index-state: 2020-05-15T00:00:00Z
+packages: free-category.cabal
+	  examples/examples.cabal
diff --git a/default.nix b/default.nix
index a2fb5c2..e11a414 100644
--- a/default.nix
+++ b/default.nix
@@ -1,50 +1,41 @@
 { compiler ? "ghc865"
-, haddock ? true
-, test ? true
-, benchmarks ? false
-, dev ? true
 }:
-with builtins;
-let
-  nixpkgs = import ./nix/nixpkgs.nix { inherit compiler; };
+let compiler-nix-name = compiler;
 
-  pkgs = nixpkgs.haskell.packages;
-  lib = nixpkgs.haskell.lib;
-  callCabal2nix = nixpkgs.haskell.packages.${compiler}.callCabal2nix;
+    sources = import ./nix/sources.nix {};
+    iohkNix = import sources.iohk-nix {};
+    haskellNix = import sources."haskell.nix" {};
+    nixpkgs = iohkNix.nixpkgs;
+    haskell-nix = haskellNix.pkgs.haskell-nix;
 
-  doHaddock = if haddock
-    then lib.doHaddock
-    else lib.dontHaddock;
-  doTest = if test
-    then lib.doCheck
-    else lib.dontCheck;
-  doBench = if benchmarks
-    then lib.doBenchmark
-    else nixpkgs.lib.id;
-  doDev = if dev
-    then drv: lib.appendConfigureFlag drv "--ghc-option -Werror"
-    else nixpkgs.lib.id;
-  noInlineRuleShadowing = if compiler == "ghc802"
-    then drv: lib.appendConfigureFlag drv "--ghc-option -Wno-inline-rule-shadowing"
-    else nixpkgs.lib.id;
-  docNoSeprateOutput = drv: lib.overrideCabal drv (drv: { enableSeparateDocOutput = false; });
-  srcFilter = src: path: type:
-    let relPath = nixpkgs.lib.removePrefix (toString src + "/") (toString path);
-    in 
-       nixpkgs.lib.hasPrefix "src" relPath
-    || nixpkgs.lib.hasPrefix "test" relPath
-    || nixpkgs.lib.hasPrefix "bench" relPath
-    || nixpkgs.lib.any
-        (a: a == relPath)
-        [ "Setup.hs" "cabal.project" "ChangeLog.md" "free-category.cabal" "LICENSE"];
+    # package set
+    pkgs = import nixpkgs
+              { config = haskellNix.config;
+                overlays = [ (_: _: { inherit freeCategoryPackages; }) ];
+              };
+    lib = pkgs.lib;
 
-  free-category = noInlineRuleShadowing(docNoSeprateOutput(doDev(doHaddock(doTest(doBench(
-    lib.overrideCabal (callCabal2nix "free-category" ./.  {})
-      (drv: {src = nixpkgs.lib.cleanSourceWith { filter = srcFilter drv.src; src = drv.src; };})
-  ))))));
-  examples = docNoSeprateOutput(doDev(doHaddock(doTest(doBench(
-    lib.overrideCabal (callCabal2nix "examples" ./examples { inherit free-category; })
-      (drv: {src = nixpkgs.lib.sourceFilesBySuffices drv.src [ ".hs" "LICENSE" "ChangeLog.md" "examples.cabal" ];})
-  )))));
+    # 'cleanGit' cleans a source directory based on the files known by git
+    src = haskell-nix.haskellLib.cleanGit {
+      name = "free-algebras";
+      src = ./.;
+    };
 
-in { inherit free-category examples; }
+    # unmodified packages
+    projectPackages = lib.attrNames
+      (haskell-nix.haskellLib.selectProjectPackages
+      (haskell-nix.cabalProject { inherit src compiler-nix-name; }));
+
+    # set GHC options
+    freeCategoryPackages = haskell-nix.cabalProject {
+        inherit src compiler-nix-name;
+        modules =
+          [
+            { packages =
+                lib.genAttrs
+                  projectPackages
+                  (name: { configureFlags = [ "--ghc-option=-Werror" ]; });
+            }
+          ];
+      };
+in pkgs.freeCategoryPackages
diff --git a/free-category.cabal b/free-category.cabal
index 86799ab..d8135db 100644
--- a/free-category.cabal
+++ b/free-category.cabal
@@ -1,3 +1,4 @@
+cabal-version:  >= 2.0
 name:           free-category
 version:        0.0.4.1
 synopsis:       efficient data types for free categories and arrows
@@ -15,7 +16,6 @@ copyright:      (c) 2018-2019 Marcin Szamotulski
 license:        MPL-2.0
 license-file:   LICENSE
 build-type:     Simple
-cabal-version:  >= 1.10
 extra-source-files:
     ChangeLog.md
     README.md
diff --git a/nix/ansi-terminal-0.6.3.1.nix b/nix/ansi-terminal-0.6.3.1.nix
deleted file mode 100644
index babbde7..0000000
--- a/nix/ansi-terminal-0.6.3.1.nix
+++ /dev/null
@@ -1,12 +0,0 @@
-{ mkDerivation, base, stdenv }:
-mkDerivation {
-  pname = "ansi-terminal";
-  version = "0.6.3.1";
-  sha256 = "458f98e0c9217897f0ff07f730cfc3ed380089936fb31942aec31bb336608095";
-  isLibrary = true;
-  isExecutable = true;
-  libraryHaskellDepends = [ base ];
-  homepage = "https://github.com/feuerbach/ansi-terminal";
-  description = "Simple ANSI terminal support, with Windows compatibility";
-  license = stdenv.lib.licenses.bsd3;
-}
diff --git a/nix/async-2.1.1.1.nix b/nix/async-2.1.1.1.nix
deleted file mode 100644
index fa008e5..0000000
--- a/nix/async-2.1.1.1.nix
+++ /dev/null
@@ -1,15 +0,0 @@
-{ mkDerivation, base, HUnit, stdenv, stm, test-framework
-, test-framework-hunit
-}:
-mkDerivation {
-  pname = "async";
-  version = "2.1.1.1";
-  sha256 = "cd83e471466ea6885b2e8fb60f452db3ac3fdf3ea2d6370aa1e071ebc37544e2";
-  libraryHaskellDepends = [ base stm ];
-  testHaskellDepends = [
-    base HUnit test-framework test-framework-hunit
-  ];
-  homepage = "https://github.com/simonmar/async";
-  description = "Run IO operations asynchronously and wait for their results";
-  license = stdenv.lib.licenses.bsd3;
-}
diff --git a/nix/cabal-doctest-1.0.7.nix b/nix/cabal-doctest-1.0.7.nix
deleted file mode 100644
index db30cec..0000000
--- a/nix/cabal-doctest-1.0.7.nix
+++ /dev/null
@@ -1,10 +0,0 @@
-{ mkDerivation, base, Cabal, directory, filepath, stdenv }:
-mkDerivation {
-  pname = "cabal-doctest";
-  version = "1.0.7";
-  sha256 = "1080ed5f7eca48621853178d701dbee209698eb357957e00a56da6d635a7adec";
-  libraryHaskellDepends = [ base Cabal directory filepath ];
-  homepage = "https://github.com/phadej/cabal-doctest";
-  description = "A Setup.hs helper for doctests running";
-  license = stdenv.lib.licenses.bsd3;
-}
diff --git a/nix/concurrent-output-1.9.2.nix b/nix/concurrent-output-1.9.2.nix
deleted file mode 100644
index 8f70601..0000000
--- a/nix/concurrent-output-1.9.2.nix
+++ /dev/null
@@ -1,14 +0,0 @@
-{ mkDerivation, ansi-terminal, async, base, directory, exceptions
-, process, stdenv, stm, terminal-size, text, transformers, unix
-}:
-mkDerivation {
-  pname = "concurrent-output";
-  version = "1.9.2";
-  sha256 = "3bc2423adb5cdae14278e718b1335363cc21cd74b370d47dc4e07d2287b2d9f3";
-  libraryHaskellDepends = [
-    ansi-terminal async base directory exceptions process stm
-    terminal-size text transformers unix
-  ];
-  description = "Ungarble output from several threads or commands";
-  license = stdenv.lib.licenses.bsd2;
-}
diff --git a/nix/exceptions-0.8.3.nix b/nix/exceptions-0.8.3.nix
deleted file mode 100644
index 214203a..0000000
--- a/nix/exceptions-0.8.3.nix
+++ /dev/null
@@ -1,21 +0,0 @@
-{ mkDerivation, base, mtl, QuickCheck, stdenv, stm
-, template-haskell, test-framework, test-framework-quickcheck2
-, transformers, transformers-compat
-}:
-mkDerivation {
-  pname = "exceptions";
-  version = "0.8.3";
-  sha256 = "4d6ad97e8e3d5dc6ce9ae68a469dc2fd3f66e9d312bc6faa7ab162eddcef87be";
-  revision = "5";
-  editedCabalFile = "1kfgp41i6mfz9gjczp3flvqxfhnznd81rwldv8j05807n6mnqqii";
-  libraryHaskellDepends = [
-    base mtl stm template-haskell transformers transformers-compat
-  ];
-  testHaskellDepends = [
-    base mtl QuickCheck stm template-haskell test-framework
-    test-framework-quickcheck2 transformers transformers-compat
-  ];
-  homepage = "http://github.com/ekmett/exceptions/";
-  description = "Extensible optionally-pure exceptions";
-  license = stdenv.lib.licenses.bsd3;
-}
diff --git a/nix/free-algebras.nix b/nix/free-algebras.nix
deleted file mode 100644
index 321bf3c..0000000
--- a/nix/free-algebras.nix
+++ /dev/null
@@ -1,19 +0,0 @@
-{ mkDerivation, base, containers, data-fix, dlist, free, groups
-, hedgehog, kan-extensions, mtl, stdenv, transformers
-}:
-mkDerivation {
-  pname = "free-algebras";
-  version = "0.0.8.0";
-  sha256 = "47eafb41c60ca509de7078f5d283bd88cf7e1ff81ddb49965d67a9e0e9969b43";
-  libraryHaskellDepends = [
-    base containers data-fix dlist free groups kan-extensions mtl
-    transformers
-  ];
-  testHaskellDepends = [
-    base containers data-fix dlist free groups hedgehog kan-extensions
-    mtl transformers
-  ];
-  homepage = "https://github.com/coot/free-algebras#readme";
-  description = "Free algebras in Haskell";
-  license = stdenv.lib.licenses.mpl20;
-}
diff --git a/nix/haskell-src-exts-1.21.1.nix b/nix/haskell-src-exts-1.21.1.nix
deleted file mode 100644
index b057fe8..0000000
--- a/nix/haskell-src-exts-1.21.1.nix
+++ /dev/null
@@ -1,19 +0,0 @@
-{ mkDerivation, array, base, containers, directory, filepath
-, ghc-prim, happy, mtl, pretty, pretty-show, smallcheck, stdenv
-, tasty, tasty-golden, tasty-smallcheck
-}:
-mkDerivation {
-  pname = "haskell-src-exts";
-  version = "1.21.1";
-  sha256 = "2ec911614325f1e5eef7e5bef15c08b8265931d69da3ce566af109d486453e60";
-  libraryHaskellDepends = [ array base ghc-prim pretty ];
-  libraryToolDepends = [ happy ];
-  testHaskellDepends = [
-    base containers directory filepath mtl pretty-show smallcheck tasty
-    tasty-golden tasty-smallcheck
-  ];
-  doCheck = false;
-  homepage = "https://github.com/haskell-suite/haskell-src-exts";
-  description = "Manipulating Haskell source: abstract syntax, lexer, parser, and pretty-printer";
-  license = stdenv.lib.licenses.bsd3;
-}
diff --git a/nix/hedgehog-1.0.1.nix b/nix/hedgehog-1.0.1.nix
deleted file mode 100644
index 974b3bb..0000000
--- a/nix/hedgehog-1.0.1.nix
+++ /dev/null
@@ -1,26 +0,0 @@
-{ mkDerivation, ansi-terminal, async, base, bytestring
-, concurrent-output, containers, directory, exceptions, fail
-, lifted-async, mmorph, monad-control, mtl, pretty-show, primitive
-, random, resourcet, semigroups, stdenv, stm, template-haskell
-, text, time, transformers, transformers-base, wl-pprint-annotated
-}:
-mkDerivation {
-  pname = "hedgehog";
-  version = "1.0.1";
-  sha256 = "202ea01fe1f30890307bd8c1ceaa0d24f988472e8371237271636abbf08487e1";
-  revision = "1";
-  editedCabalFile = "0dq3ry7py2wsiwxar11zbvm3xmifm92nx4bh61lqxzmpwyyiwnxn";
-  libraryHaskellDepends = [
-    ansi-terminal async base bytestring concurrent-output containers
-    directory exceptions fail lifted-async mmorph monad-control mtl
-    pretty-show primitive random resourcet semigroups stm
-    template-haskell text time transformers transformers-base
-    wl-pprint-annotated
-  ];
-  testHaskellDepends = [
-    base containers mmorph mtl pretty-show semigroups text transformers
-  ];
-  homepage = "https://hedgehog.qa";
-  description = "Release with confidence";
-  license = stdenv.lib.licenses.bsd3;
-}
diff --git a/nix/hoopl-3.10.2.2.nix b/nix/hoopl-3.10.2.2.nix
deleted file mode 100644
index e260f97..0000000
--- a/nix/hoopl-3.10.2.2.nix
+++ /dev/null
@@ -1,18 +0,0 @@
-{ mkDerivation, base, containers, filepath, mtl, parsec, stdenv
-, test-framework, test-framework-hunit
-}:
-mkDerivation {
-  pname = "hoopl";
-  version = "3.10.2.2";
-  sha256 = "097b1316d5f1c8ffe71133223209eb2b095fe13f43dc01d1fe43fd8a545a2b97";
-  revision = "2";
-  editedCabalFile = "0j6pz4jzhvviyrhhn1j22ikmjvzrg60nzvq26lbpkcb6y4q6rlyx";
-  libraryHaskellDepends = [ base containers ];
-  testHaskellDepends = [
-    base containers filepath mtl parsec test-framework
-    test-framework-hunit
-  ];
-  homepage = "https://github.com/haskell/hoopl";
-  description = "A library to support dataflow analysis and optimization";
-  license = stdenv.lib.licenses.bsd3;
-}
diff --git a/nix/lifted-async-0.9.3.3.nix b/nix/lifted-async-0.9.3.3.nix
deleted file mode 100644
index 5906e05..0000000
--- a/nix/lifted-async-0.9.3.3.nix
+++ /dev/null
@@ -1,20 +0,0 @@
-{ mkDerivation, async, base, constraints, criterion, deepseq, HUnit
-, lifted-base, monad-control, mtl, stdenv, tasty, tasty-hunit
-, tasty-th, transformers-base
-}:
-mkDerivation {
-  pname = "lifted-async";
-  version = "0.9.3.3";
-  sha256 = "ae37d9cab3dd21aa65e4722c5268585d2d555fea1e8870920e7e567160240dbf";
-  libraryHaskellDepends = [
-    async base constraints lifted-base monad-control transformers-base
-  ];
-  testHaskellDepends = [
-    async base HUnit lifted-base monad-control mtl tasty tasty-hunit
-    tasty-th
-  ];
-  benchmarkHaskellDepends = [ async base criterion deepseq ];
-  homepage = "https://github.com/maoe/lifted-async";
-  description = "Run lifted IO operations asynchronously and wait for their results";
-  license = stdenv.lib.licenses.bsd3;
-}
diff --git a/nix/nixpkgs.nix b/nix/nixpkgs.nix
deleted file mode 100644
index b248e87..0000000
--- a/nix/nixpkgs.nix
+++ /dev/null
@@ -1,45 +0,0 @@
-{ compiler ? "ghc865" }:
-with builtins;
-let
-  rev = if   compiler == "ghc802"
-          || compiler == "ghc822"
-          || compiler == "ghc844"
-    then "722fcbbb80b2142583e9266efe77992f8e32ac4c"
-    else "a2f9bcd1328de9c043d7425e45d06fc05e0b7929";
-  url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
-  config =
-    { packageOverrides = super:
-      let self = super.pkgs;
-          lib = super.haskell.lib;
-          overrides = self: super: {
-            free-algebras = super.callPackage ./free-algebras.nix {};
-          };
-      in {
-        haskell = super.haskell // {
-          packages = super.haskell.packages // {
-            ghc881 = super.haskell.packages.ghc881.override {
-              overrides = self: super: overrides self super // {
-                cabal-doctest = super.callPackage ./cabal-doctest-1.0.7.nix {};
-                haskell-src-exts = super.callPackage ./haskell-src-exts-1.21.1.nix {};
-                hedgehog = super.callPackage ./hedgehog-1.0.1.nix {};
-              };
-            };
-            ghc865 = super.haskell.packages.ghc865.override { inherit overrides; };
-            ghc844 = super.haskell.packages.ghc844.override { inherit overrides; };
-            ghc822 = super.haskell.packages.ghc822.override { inherit overrides; };
-            ghc802 = super.haskell.packages.ghc802.override {
-              overrides = self: super: overrides self super // {
-                ansi-terminal = super.callPackage ./ansi-terminal-0.6.3.1.nix {};
-                async = super.callPackage ./async-2.1.1.1.nix {};
-                lifted-async = super.callPackage ./lifted-async-0.9.3.3.nix {};
-                exceptions = super.callPackage ./exceptions-0.8.3.nix {};
-                stm = super.callPackage ./stm-2.4.5.1.nix {};
-                concurrent-output = super.callPackage ./concurrent-output-1.9.2.nix {};
-              };
-            };
-          };
-        };
-      };
-    };
-  nixpkgs = import (fetchTarball { inherit url; }) { inherit config; };
-in nixpkgs
diff --git a/nix/sources.json b/nix/sources.json
new file mode 100644
index 0000000..bf83668
--- /dev/null
+++ b/nix/sources.json
@@ -0,0 +1,38 @@
+{
+    "haskell.nix": {
+        "branch": "master",
+        "description": "Alternative Haskell Infrastructure for Nixpkgs",
+        "homepage": "https://input-output-hk.github.io/haskell.nix",
+        "owner": "input-output-hk",
+        "repo": "haskell.nix",
+        "rev": "32baaac4d7858ae3349c1f6d1452371f998c7f49",
+        "sha256": "0g0jh0jwips3qyxs7rb1d4qvqs6rralnq318x5x2wvy1mkzvxvlg",
+        "type": "tarball",
+        "url": "https://github.com/input-output-hk/haskell.nix/archive/32baaac4d7858ae3349c1f6d1452371f998c7f49.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "iohk-nix": {
+        "branch": "master",
+        "description": "nix scripts shared across projects",
+        "homepage": null,
+        "owner": "input-output-hk",
+        "repo": "iohk-nix",
+        "rev": "00788ac974a517846dba09bd5e095757011cb083",
+        "sha256": "0lxffzd31a359d7j9q0vwaj6d5lq0ka5df6g0g30rm91sw7jkpnj",
+        "type": "tarball",
+        "url": "https://github.com/input-output-hk/iohk-nix/archive/00788ac974a517846dba09bd5e095757011cb083.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "niv": {
+        "branch": "master",
+        "description": "Easy dependency management for Nix projects",
+        "homepage": "https://github.com/nmattia/niv",
+        "owner": "nmattia",
+        "repo": "niv",
+        "rev": "372f96bff217a7a019de27667d04118cffa9841b",
+        "sha256": "1l0z6162zw60pdcdj03aq64qgf1vyzmf24i9pxca64i4sprl1b7p",
+        "type": "tarball",
+        "url": "https://github.com/nmattia/niv/archive/372f96bff217a7a019de27667d04118cffa9841b.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    }
+}
diff --git a/nix/sources.nix b/nix/sources.nix
new file mode 100644
index 0000000..8a725cb
--- /dev/null
+++ b/nix/sources.nix
@@ -0,0 +1,134 @@
+# This file has been generated by Niv.
+
+let
+
+  #
+  # The fetchers. fetch_<type> fetches specs of type <type>.
+  #
+
+  fetch_file = pkgs: spec:
+    if spec.builtin or true then
+      builtins_fetchurl { inherit (spec) url sha256; }
+    else
+      pkgs.fetchurl { inherit (spec) url sha256; };
+
+  fetch_tarball = pkgs: spec:
+    if spec.builtin or true then
+      builtins_fetchTarball { inherit (spec) url sha256; }
+    else
+      pkgs.fetchzip { inherit (spec) url sha256; };
+
+  fetch_git = spec:
+    builtins.fetchGit { url = spec.repo; inherit (spec) rev ref; };
+
+  fetch_builtin-tarball = spec:
+    builtins.trace
+      ''
+        WARNING:
+          The niv type "builtin-tarball" will soon be deprecated. You should
+          instead use `builtin = true`.
+
+          $ niv modify <package> -a type=tarball -a builtin=true
+      ''
+      builtins_fetchTarball { inherit (spec) url sha256; };
+
+  fetch_builtin-url = spec:
+    builtins.trace
+      ''
+        WARNING:
+          The niv type "builtin-url" will soon be deprecated. You should
+          instead use `builtin = true`.
+
+          $ niv modify <package> -a type=file -a builtin=true
+      ''
+      (builtins_fetchurl { inherit (spec) url sha256; });
+
+  #
+  # Various helpers
+  #
+
+  # The set of packages used when specs are fetched using non-builtins.
+  mkPkgs = sources:
+    let
+      sourcesNixpkgs =
+        import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; }) {};
+      hasNixpkgsPath = builtins.any (x: x.prefix == "nixpkgs") builtins.nixPath;
+      hasThisAsNixpkgsPath = <nixpkgs> == ./.;
+    in
+      if builtins.hasAttr "nixpkgs" sources
+      then sourcesNixpkgs
+      else if hasNixpkgsPath && ! hasThisAsNixpkgsPath then
+        import <nixpkgs> {}
+      else
+        abort
+          ''
+            Please specify either <nixpkgs> (through -I or NIX_PATH=nixpkgs=...) or
+            add a package called "nixpkgs" to your sources.json.
+          '';
+
+  # The actual fetching function.
+  fetch = pkgs: name: spec:
+
+    if ! builtins.hasAttr "type" spec then
+      abort "ERROR: niv spec ${name} does not have a 'type' attribute"
+    else if spec.type == "file" then fetch_file pkgs spec
+    else if spec.type == "tarball" then fetch_tarball pkgs spec
+    else if spec.type == "git" then fetch_git spec
+    else if spec.type == "builtin-tarball" then fetch_builtin-tarball spec
+    else if spec.type == "builtin-url" then fetch_builtin-url spec
+    else
+      abort "ERROR: niv spec ${name} has unknown type ${builtins.toJSON spec.type}";
+
+  # Ports of functions for older nix versions
+
+  # a Nix version of mapAttrs if the built-in doesn't exist
+  mapAttrs = builtins.mapAttrs or (
+    f: set: with builtins;
+    listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set))
+  );
+
+  # fetchTarball version that is compatible between all the versions of Nix
+  builtins_fetchTarball = { url, sha256 }@attrs:
+    let
+      inherit (builtins) lessThan nixVersion fetchTarball;
+    in
+      if lessThan nixVersion "1.12" then
+        fetchTarball { inherit url; }
+      else
+        fetchTarball attrs;
+
+  # fetchurl version that is compatible between all the versions of Nix
+  builtins_fetchurl = { url, sha256 }@attrs:
+    let
+      inherit (builtins) lessThan nixVersion fetchurl;
+    in
+      if lessThan nixVersion "1.12" then
+        fetchurl { inherit url; }
+      else
+        fetchurl attrs;
+
+  # Create the final "sources" from the config
+  mkSources = config:
+    mapAttrs (
+      name: spec:
+        if builtins.hasAttr "outPath" spec
+        then abort
+          "The values in sources.json should not have an 'outPath' attribute"
+        else
+          spec // { outPath = fetch config.pkgs name spec; }
+    ) config.sources;
+
+  # The "config" used by the fetchers
+  mkConfig =
+    { sourcesFile ? ./sources.json
+    , sources ? builtins.fromJSON (builtins.readFile sourcesFile)
+    , pkgs ? mkPkgs sources
+    }: rec {
+      # The sources, i.e. the attribute set of spec name to spec
+      inherit sources;
+
+      # The "pkgs" (evaluated nixpkgs) to use for e.g. non-builtin fetchers
+      inherit pkgs;
+    };
+in
+mkSources (mkConfig {}) // { __functor = _: settings: mkSources (mkConfig settings); }
diff --git a/nix/stm-2.4.5.1.nix b/nix/stm-2.4.5.1.nix
deleted file mode 100644
index 3f61ee0..0000000
--- a/nix/stm-2.4.5.1.nix
+++ /dev/null
@@ -1,10 +0,0 @@
-{ mkDerivation, array, base, stdenv }:
-mkDerivation {
-  pname = "stm";
-  version = "2.4.5.1";
-  sha256 = "6cf0c280062736c9980ba1c2316587648b8e9d4e4ecc5aed16a41979c0a3a3f4";
-  libraryHaskellDepends = [ array base ];
-  homepage = "https://wiki.haskell.org/Software_transactional_memory";
-  description = "Software Transactional Memory";
-  license = stdenv.lib.licenses.bsd3;
-}