Update nixpkgs via added niv

nix/default.nix

@@ -1,12 +1,13 @@
 /**
  * This is the entry-point for all nix execution in this project.
  */
-{ nixpkgsSrc ? ./nixpkgs.nix
+{ sources ? import ./sources.nix
+, nixpkgsSrc ? sources.nixpkgs
 , system ? null
 , ...
 }:
 
-import (import ./nixpkgs.nix) ({
+import nixpkgsSrc ({
   # Makes the config pure as well. See <nixpkgs>/top-level/impure.nix:
   config = {
   };

nix/nixpkgs.nix

@@ -1,5 +0,0 @@
-# to update: $ nix-prefetch-url --unpack url
-builtins.fetchTarball {
-  url = "https://github.com/NixOS/nixpkgs/archive/bd5e8f35c2e9d1ddc9cd2fea7a23563336d54acb.tar.gz";
-  sha256 = "1wnzqqijrwf797nb234q10zb1h7086njradkkrx3a15b303grsw4";
-}

nix/overlay.nix

@@ -3,6 +3,8 @@ let
   inherit (self.arion-project) haskellPkgs;
   inherit (super) lib;
 
+  sources = import ./sources.nix;
+
 in
 {
 
@@ -19,7 +21,10 @@ in
         haskellPkgs.ghcid
         super.docker-compose
         (import ~/h/ghcide-nix {}).ghcide-ghc864
+        self.niv
       ];
     };
   };
+
+  inherit (import (sources.niv) {}) niv;
 }

nix/sources.json

@@ -0,0 +1,26 @@
+{
+    "niv": {
+        "branch": "master",
+        "description": "Easy dependency management for Nix projects",
+        "homepage": "https://github.com/nmattia/niv",
+        "owner": "nmattia",
+        "repo": "niv",
+        "rev": "1dd094156b249586b66c16200ecfd365c7428dc0",
+        "sha256": "1b2vjnn8iac5iiqszjc2v1s1ygh0yri998c0k3s4x4kn0dsqik21",
+        "type": "tarball",
+        "url": "https://github.com/nmattia/niv/archive/1dd094156b249586b66c16200ecfd365c7428dc0.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "nixpkgs": {
+        "branch": "nixos-19.03",
+        "description": "A read-only mirror of NixOS/nixpkgs tracking the released channels. Send issues and PRs to",
+        "homepage": "https://github.com/NixOS/nixpkgs",
+        "owner": "NixOS",
+        "repo": "nixpkgs-channels",
+        "rev": "6420e2649fa9e267481fb78e602022dab9d1dcd1",
+        "sha256": "1z3hx7gp8nxk3fspi8vik3j9zxpajj3s7nxvjvx3b5igndxwbp74",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs-channels/archive/6420e2649fa9e267481fb78e602022dab9d1dcd1.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    }
+}

nix/sources.nix

@@ -0,0 +1,93 @@
+# This file has been generated by Niv.
+
+# A record, from name to path, of the third-party packages
+with rec
+{
+  pkgs =
+    if hasNixpkgsPath
+    then
+        if hasThisAsNixpkgsPath
+        then import (builtins_fetchTarball { inherit (sources_nixpkgs) url sha256; }) {}
+        else import <nixpkgs> {}
+    else
+        import (builtins_fetchTarball { inherit (sources_nixpkgs) url sha256; }) {};
+
+  sources_nixpkgs =
+    if builtins.hasAttr "nixpkgs" sources
+    then sources.nixpkgs
+    else abort
+    ''
+        Please specify either <nixpkgs> (through -I or NIX_PATH=nixpkgs=...) or
+        add a package called "nixpkgs" to your sources.json.
+    '';
+
+  # 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;
+
+  # A wrapper around pkgs.fetchzip that has inspectable arguments,
+  # annoyingly this means we have to specify them
+  fetchzip = { url, sha256 }@attrs: pkgs.fetchzip attrs;
+
+  # A wrapper around pkgs.fetchurl that has inspectable arguments,
+  # annoyingly this means we have to specify them
+  fetchurl = { url, sha256 }@attrs: pkgs.fetchurl attrs;
+
+  hasNixpkgsPath = (builtins.tryEval <nixpkgs>).success;
+  hasThisAsNixpkgsPath =
+    (builtins.tryEval <nixpkgs>).success && <nixpkgs> == ./.;
+
+  sources = builtins.fromJSON (builtins.readFile ./sources.json);
+
+  mapAttrs = builtins.mapAttrs or
+    (f: set: with builtins;
+      listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set)));
+
+  # borrowed from nixpkgs
+  functionArgs = f: f.__functionArgs or (builtins.functionArgs f);
+  callFunctionWith = autoArgs: f: args:
+    let auto = builtins.intersectAttrs (functionArgs f) autoArgs;
+    in f (auto // args);
+
+  getFetcher = spec:
+    let fetcherName =
+      if builtins.hasAttr "type" spec
+      then builtins.getAttr "type" spec
+      else "builtin-tarball";
+    in builtins.getAttr fetcherName {
+      "tarball" = fetchzip;
+      "builtin-tarball" = builtins_fetchTarball;
+      "file" = fetchurl;
+      "builtin-url" = builtins_fetchurl;
+    };
+};
+# NOTE: spec must _not_ have an "outPath" attribute
+mapAttrs (_: spec:
+  if builtins.hasAttr "outPath" spec
+  then abort
+    "The values in sources.json should not have an 'outPath' attribute"
+  else
+    if builtins.hasAttr "url" spec && builtins.hasAttr "sha256" spec
+    then
+      spec //
+      { outPath = callFunctionWith spec (getFetcher spec) { }; }
+    else spec
+  ) sources