diff --git a/default.nix b/default.nix
index 5505951c34..7b1b90bd1c 100644
--- a/default.nix
+++ b/default.nix
@@ -2,10 +2,9 @@ let haskellNix = {
       checkMaterialization ? false,  # Allows us to easily switch on materialization checking
       defaultCompilerNixName ? null, # Quick way to override the default compiler e.g. "ghc883"
       system ? builtins.currentSystem,
-      ... }: rec {
-    sources = {
-      inherit (import ./nixpkgs/default.nix) nixpkgs-1909 nixpkgs-2003 nixpkgs-default;
-    };
+      sourcesOverride ? {},
+      ... }@args: rec {
+    sources = (import ./nix/sources.nix) // sourcesOverride;
 
     config   = import ./config.nix;
     overlays = [ allOverlays.combined ] ++ (
@@ -29,7 +28,7 @@ let haskellNix = {
         )]
         else []
     );
-    allOverlays = import ./overlays;
+    allOverlays = import ./overlays args;
     nixpkgsArgs = { inherit config overlays system; };
     pkgs = import sources.nixpkgs-default nixpkgsArgs;
   };
diff --git a/nix/sources.json b/nix/sources.json
new file mode 100644
index 0000000000..8ec5d0441d
--- /dev/null
+++ b/nix/sources.json
@@ -0,0 +1,50 @@
+{
+    "niv": {
+        "branch": "master",
+        "description": "Easy dependency management for Nix projects",
+        "homepage": "https://github.com/nmattia/niv",
+        "owner": "nmattia",
+        "repo": "niv",
+        "rev": "f73bf8d584148677b01859677a63191c31911eae",
+        "sha256": "0jlmrx633jvqrqlyhlzpvdrnim128gc81q5psz2lpp2af8p8q9qs",
+        "type": "tarball",
+        "url": "https://github.com/nmattia/niv/archive/f73bf8d584148677b01859677a63191c31911eae.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "nixpkgs-1909": {
+        "branch": "nixpkgs-19.09-darwin",
+        "description": "Nix Packages collection",
+        "homepage": null,
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "9237a09d8edbae9951a67e9a3434a07ef94035b7",
+        "sha256": "05bizymljzzd665bpsjbhxamcgzq7bkjjzjfapkl2nicy774ak4x",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/9237a09d8edbae9951a67e9a3434a07ef94035b7.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "nixpkgs-2003": {
+        "branch": "nixpkgs-20.03-darwin",
+        "description": "Nix Packages collection",
+        "homepage": null,
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "cac363c661817666e43d047addfaa722610d425f",
+        "sha256": "0fi8hgddy8qh2jrsa40jw7jxnr5lrhq2ji6a2xbndllivhzc31kf",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/cac363c661817666e43d047addfaa722610d425f.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "nixpkgs-default": {
+        "branch": "nixpkgs-20.03-darwin",
+        "description": "Nix Packages collection",
+        "homepage": null,
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "cac363c661817666e43d047addfaa722610d425f",
+        "sha256": "0fi8hgddy8qh2jrsa40jw7jxnr5lrhq2ji6a2xbndllivhzc31kf",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/cac363c661817666e43d047addfaa722610d425f.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 0000000000..8a725cb4e7
--- /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/nixpkgs/default.nix b/nixpkgs/default.nix
index 51bdd82966..867d2d7fa9 100644
--- a/nixpkgs/default.nix
+++ b/nixpkgs/default.nix
@@ -1,16 +1,4 @@
-# see ../docs/dev/nixpkgs-pin.md
-let
-  fetch = jsonFile:
-    with builtins;
-    let spec = fromJSON (readFile jsonFile);
-    in fetchTarball {
-      name = "nixpkgs";
-      inherit (spec) sha256;
-      url = "${spec.url}/archive/${spec.rev}.tar.gz";
-    };
-in
+# This file is for backwards compatibility only
 {
-  nixpkgs-2003 = fetch (./. + "/release-20.03.json");
-  nixpkgs-1909 = fetch (./. + "/release-19.09.json");
-  nixpkgs-default = fetch (./. + "/github.json");
+  inherit (import ../nix/sources.nix) nixpkgs-2003 nixpkgs-1909 nixpkgs-default;
 }
diff --git a/nixpkgs/github.json b/nixpkgs/github.json
deleted file mode 100644
index e10e117e58..0000000000
--- a/nixpkgs/github.json
+++ /dev/null
@@ -1,9 +0,0 @@
-{
-  "url": "https://github.com/NixOS/nixpkgs",
-  "rev": "cac363c661817666e43d047addfaa722610d425f",
-  "date": "2020-03-24T13:44:58+01:00",
-  "sha256": "0fi8hgddy8qh2jrsa40jw7jxnr5lrhq2ji6a2xbndllivhzc31kf",
-  "fetchSubmodules": false,
-  "deepClone": false,
-  "leaveDotGit": false
-}
diff --git a/nixpkgs/hackage-src.json b/nixpkgs/hackage-src.json
deleted file mode 100644
index e69de29bb2..0000000000
diff --git a/nixpkgs/release-19.09.json b/nixpkgs/release-19.09.json
deleted file mode 100644
index b52c040394..0000000000
--- a/nixpkgs/release-19.09.json
+++ /dev/null
@@ -1,9 +0,0 @@
-{
-  "url": "https://github.com/NixOS/nixpkgs",
-  "rev": "9237a09d8edbae9951a67e9a3434a07ef94035b7",
-  "date": "2020-04-19T11:10:38+05:30",
-  "sha256": "05bizymljzzd665bpsjbhxamcgzq7bkjjzjfapkl2nicy774ak4x",
-  "fetchSubmodules": false,
-  "deepClone": false,
-  "leaveDotGit": false
-}
diff --git a/nixpkgs/release-20.03.json b/nixpkgs/release-20.03.json
deleted file mode 100644
index e10e117e58..0000000000
--- a/nixpkgs/release-20.03.json
+++ /dev/null
@@ -1,9 +0,0 @@
-{
-  "url": "https://github.com/NixOS/nixpkgs",
-  "rev": "cac363c661817666e43d047addfaa722610d425f",
-  "date": "2020-03-24T13:44:58+01:00",
-  "sha256": "0fi8hgddy8qh2jrsa40jw7jxnr5lrhq2ji6a2xbndllivhzc31kf",
-  "fetchSubmodules": false,
-  "deepClone": false,
-  "leaveDotGit": false
-}
diff --git a/overlays/default.nix b/overlays/default.nix
index 40f0c68756..00eeade884 100644
--- a/overlays/default.nix
+++ b/overlays/default.nix
@@ -1,9 +1,11 @@
+args:
+
 let
   overlays = {
     wine = import ./wine.nix;
     #ghcjs = import ./ghcjs-asterius-triple.nix;
     #python = import ./python.nix;
-    haskell = import ./haskell.nix;
+    haskell = import ./haskell.nix args;
     hackage-quirks = import ./hackage-quirks.nix;
     bootstrap = import ./bootstrap.nix;
     ghc = import ./ghc.nix;
diff --git a/overlays/haskell.nix b/overlays/haskell.nix
index 958029abda..c267163669 100644
--- a/overlays/haskell.nix
+++ b/overlays/haskell.nix
@@ -1,3 +1,5 @@
+{ sourcesOverride ? {}
+,  ... }:
 # The haskell.nix infrastructure
 #
 # for hygenic reasons we'll use haskell-nix as a prefix.
@@ -10,6 +12,26 @@ final: prev: {
         # overlays.
         defaultModules = [];
 
+        # Niv based source pins.  See https://github.com/nmattia/niv#niv
+        # for details on how to update this using the niv tool
+        # or edit nix/sources.json manually if you prefer.
+        sources = {
+          # Hackage and stackage still updated by scripts
+          # that predate our use of niv.  We have moved them
+          # here though so that you can still use the
+          # sourcesOverride arg or `niv add` to replace them.
+          hackage = fetchExternal {
+            name     = "hackage-exprs-source";
+            specJSON = hackageSourceJSON;
+            override = "hackage";
+          };
+          stackage = fetchExternal {
+            name     = "stackage-snapshot-source";
+            specJSON = stackageSourceJSON;
+            override = "stackage";
+          };
+        } // (import ../nix/sources.nix) // sourcesOverride;
+
         # We provide a `callPackage` function to consumers for
         # convenience.  We will however refrain from using it
         # here and be explicit about imports and dependencies.
@@ -37,11 +59,7 @@ final: prev: {
           cleanSourceHaskell;
 
         # All packages from Hackage as Nix expressions
-        hackageSrc = fetchExternal {
-            name     = "hackage-exprs-source";
-            specJSON = hackageSourceJSON;
-            override = "hackage";
-        };
+        hackageSrc = sources.hackage;
         hackage = import hackageSrc;
 
         # Contains the hashes of the cabal 01-index.tar.gz for given
@@ -49,12 +67,7 @@ final: prev: {
         indexStateHashesPath = hackageSrc + "/index-state-hashes.nix";
 
         # The set of all Stackage snapshots
-        stackageSrc = fetchExternal {
-            name     = "stackage-snapshot-source";
-            specJSON = stackageSourceJSON;
-            override = "stackage";
-        };
-
+        stackageSrc = sources.stackage;
         stackage = import stackageSrc;
 
         # Utility functions for working with the component builder.