Optionally pass Nixpkgs through entry functions (#1409)

The "builtins.fetchTarball" function is not available in some restricted
environments such as build jobs of the latest Hydra. Make it possible to
pass nixpkgs and nixpkgsStaticLinux when evaulating default.nix and
release.nix to avoid importing them internally. This does not change the
result of evaulation if no parameters are passed.

dhall-nix/release.nix

@@ -1,13 +1,6 @@
 let
-  fetchNixpkgs = import ./nix/fetchNixpkgs.nix;
-
-  nixpkgs = fetchNixpkgs {
-    rev = "1d4de0d552ae9aa66a5b8dee5fb0650a4372d148";
-
-    sha256 = "09qx58dp1kbj7cpzp8ahbqfbbab1frb12sh1qng87rybcaz0dz01";
-
-    outputSha256 = "0xpqc1fhkvvv5dv1zmas2j1q27mi7j7dgyjcdh82mlgl1q63i660";
-  };
+  pinned = import ./pinnedNixpkgs.nix; in
+  inherit (pinned) nixpkgs;
 
   mass = function: names: haskellPackagesNew: haskellPackagesOld:
     let

nix/fetchNixpkgs.nix

@@ -1,50 +0,0 @@
-{ rev                             # The Git revision of nixpkgs to fetch
-, owner ? "NixOS"
-, sha256                          # The SHA256 of the downloaded data
-, outputSha256 ? null             # The SHA256 fixed-output hash
-, system ? builtins.currentSystem # This is overridable if necessary
-}:
-
-if (0 <= builtins.compareVersions builtins.nixVersion "1.12")
-
-# In Nix 1.12, we can just give a `sha256` to `builtins.fetchTarball`.
-then (
-  builtins.fetchTarball {
-    url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
-    sha256 = outputSha256;
-  })
-
-# This hack should at least work for Nix 1.11
-else (
-  (rec {
-    tarball = import <nix/fetchurl.nix> {
-      url = "https://github.com/${owner}/nixpkgs/archive/${rev}.tar.gz";
-      inherit sha256;
-    };
-
-    builtin-paths = import <nix/config.nix>;
-      
-    script = builtins.toFile "nixpkgs-unpacker" ''
-      "$coreutils/mkdir" "$out"
-      cd "$out"
-      "$gzip" --decompress < "$tarball" | "$tar" -x --strip-components=1
-    '';
-
-    nixpkgs = builtins.derivation ({
-      name = "nixpkgs-${builtins.substring 0 6 rev}";
-
-      builder = builtins.storePath builtin-paths.shell;
-
-      args = [ script ];
-
-      inherit tarball system;
-
-      tar       = builtins.storePath builtin-paths.tar;
-      gzip      = builtins.storePath builtin-paths.gzip;
-      coreutils = builtins.storePath builtin-paths.coreutils;
-    } // (if null == outputSha256 then { } else {
-      outputHashMode = "recursive";
-      outputHashAlgo = "sha256";
-      outputHash = outputSha256;
-    }));
-  }).nixpkgs)

nix/pinnedNixpkgs.nix

@@ -0,0 +1,67 @@
+let
+  fetchNixpkgs =
+    { rev # The Git revision of nixpkgs to fetch
+    , owner ? "NixOS" # Owner of the Github repository
+    , sha256 # The SHA256 hash of the unpacked archive
+    , system ? builtins.currentSystem # This is overridable if necessary
+    }:
+
+    if (0 <= builtins.compareVersions builtins.nixVersion "1.12")
+
+    # In Nix 1.12, we can just give a `sha256` to `builtins.fetchTarball`.
+    then
+      (builtins.fetchTarball {
+        url = "https://github.com/${owner}/nixpkgs/archive/${rev}.tar.gz";
+        inherit sha256;
+      })
+
+      # This hack should at least work for Nix 1.11
+    else
+      ((rec {
+        tarball = import <nix/fetchurl.nix> {
+          url = "https://github.com/${owner}/nixpkgs/archive/${rev}.tar.gz";
+          sha256 = null;
+        };
+
+        builtin-paths = import <nix/config.nix>;
+
+        script = builtins.toFile "nixpkgs-unpacker" ''
+          "$coreutils/mkdir" "$out"
+          cd "$out"
+          "$gzip" --decompress < "$tarball" | "$tar" -x --strip-components=1
+        '';
+
+        nixpkgs = builtins.derivation ({
+          name = "nixpkgs-${builtins.substring 0 6 rev}";
+
+          builder = builtins.storePath builtin-paths.shell;
+
+          args = [ script ];
+
+          inherit tarball system;
+
+          tar = builtins.storePath builtin-paths.tar;
+          gzip = builtins.storePath builtin-paths.gzip;
+          coreutils = builtins.storePath builtin-paths.coreutils;
+        } // (if null == sha256 then
+          { }
+        else {
+          outputHashMode = "recursive";
+          outputHashAlgo = "sha256";
+          outputHash = sha256;
+        }));
+      }).nixpkgs);
+in {
+
+  nixpkgs = fetchNixpkgs {
+    rev = "1d4de0d552ae9aa66a5b8dee5fb0650a4372d148";
+    sha256 = "0xpqc1fhkvvv5dv1zmas2j1q27mi7j7dgyjcdh82mlgl1q63i660";
+  };
+
+  nixpkgsStaticLinux = fetchNixpkgs {
+    owner = "nh2";
+    rev = "925aac04f4ca58aceb83beef18cb7dae0715421b";
+    sha256 = "1zr8lscjl2a5cz61f0ibyx55a94v8yyp6sjzjl2gkqjrjbg99abx";
+  };
+
+}

nix/shared.nix

@@ -1,9 +1,13 @@
 let
+  pinned = import ./pinnedNixpkgs.nix;
+  
   defaultCompiler = "ghc843";
 
 in
 
-{ compiler ? defaultCompiler
+{ nixpkgs ? pinned.nixpkgs
+, nixpkgsStaticLinux ? pinned.nixpkgsStaticLinux
+, compiler ? defaultCompiler
 , coverage ? false
 , system ? builtins.currentSystem
 }:
@@ -17,8 +21,6 @@ let
     "dhall-nix"
   ];
 
-  fetchNixpkgs = import ./fetchNixpkgs.nix;
-
   mass = function: names: haskellPackagesNew: haskellPackagesOld:
     let
       toNameValue = name: {
@@ -493,16 +495,6 @@ let
     };
   };
 
-
-
-  nixpkgs = fetchNixpkgs {
-    rev = "1d4de0d552ae9aa66a5b8dee5fb0650a4372d148";
-
-    sha256 = "09qx58dp1kbj7cpzp8ahbqfbbab1frb12sh1qng87rybcaz0dz01";
-
-    outputSha256 = "0xpqc1fhkvvv5dv1zmas2j1q27mi7j7dgyjcdh82mlgl1q63i660";
-  };
-
   pkgs = import nixpkgs {
     inherit system;
 
@@ -600,16 +592,6 @@ let
     };
   };
 
-  nixpkgsStaticLinux = fetchNixpkgs {
-    owner = "nh2";
-
-    rev = "925aac04f4ca58aceb83beef18cb7dae0715421b";
-
-    sha256 = "0zkvqzzyf5c742zcl1sqc8009dr6fr1fblz53v8gfl63hzqwj0x4";
-
-    outputSha256 = "1zr8lscjl2a5cz61f0ibyx55a94v8yyp6sjzjl2gkqjrjbg99abx";
-  };
-
   pkgsStaticLinux = import nixpkgsStaticLinux {
     config = {};
     overlays = [ overlayShared overlayStaticLinux ];

release.nix

@@ -1,23 +1,25 @@
-{ src ? { rev = ""; }, ... }:
+let pinned = import ./nix/pinnedNixpkgs.nix;
+
+in { src ? { rev = ""; }
+, nixpkgs ? pinned.nixpkgs
+, nixpkgsStaticLinux ? pinned.nixpkgsStaticLinux
+}:
 
 let
-  shared_7_10_3 =
-    import ./nix/shared.nix { compiler = "ghc7103"; };
+  callShared = args:
+    import ./nix/shared.nix ({ inherit nixpkgs nixpkgsStaticLinux; } // args);
 
-  shared_8_6_1 =
-    import ./nix/shared.nix { compiler = "ghc861"; };
+  shared_7_10_3 = callShared { compiler = "ghc7103"; };
 
-  shared_ghcjs =
-    import ./nix/shared.nix { compiler = "ghcjs"; };
+  shared_8_6_1 = callShared { compiler = "ghc861"; };
 
-  shared =
-    import ./nix/shared.nix { };
+  shared_ghcjs = callShared { compiler = "ghcjs"; };
 
-  shared_linux =
-    import ./nix/shared.nix { system = "x86_64-linux"; };
+  shared = callShared { };
 
-  coverage =
-    import ./nix/shared.nix { coverage = true; };
+  shared_linux = callShared { system = "x86_64-linux"; };
+
+  coverage = callShared { coverage = true; };
 
 in
   { dhall = shared.aggregate