Add nixpkgs.nix to all examples

On CI we set NIX_PATH to "nixpkgs=./nixpkgs.nix", so nix-shell tries to
use it from within the example's directory and fails, it tries to use
builtin bash. On macOS it is too old, so we should make sure that the
one from nixpkgs is used.

Related to tweag#176

examples/toolchains/cc/nixpkgs.json

@@ -0,0 +1,7 @@
+{
+  "owner": "NixOS",
+  "repo": "nixpkgs",
+  "branch": "21.11",
+  "rev": "506445d88e183bce80e47fc612c710eb592045ed",
+  "sha256": "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02"
+}

examples/toolchains/cc/nixpkgs.nix

@@ -0,0 +1,9 @@
+let
+  # nixpkgs-unstable as of 2021-02-19
+  spec = builtins.fromJSON (builtins.readFile ./nixpkgs.json);
+  nixpkgs = fetchTarball {
+    url = "https://github.com/${spec.owner}/${spec.repo}/archive/${spec.rev}.tar.gz";
+    sha256 = spec.sha256;
+  };
+in
+import nixpkgs

examples/toolchains/cc/shell.nix

@@ -1,6 +1,3 @@
-{ pkgs ? import (builtins.fetchTarball {
-  url = "https://github.com/NixOS/nixpkgs/archive/21.11.tar.gz";
-  sha256 = "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02";
-}) { } }:
+{ pkgs ? import ./nixpkgs.nix { } }:
 
 pkgs.mkShell { nativeBuildInputs = [ pkgs.bazel_4 ]; }

examples/toolchains/cc_with_deps/nixpkgs.json

@@ -0,0 +1,7 @@
+{
+  "owner": "NixOS",
+  "repo": "nixpkgs",
+  "branch": "21.11",
+  "rev": "506445d88e183bce80e47fc612c710eb592045ed",
+  "sha256": "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02"
+}

examples/toolchains/cc_with_deps/nixpkgs.nix

@@ -0,0 +1,9 @@
+let
+  # nixpkgs-unstable as of 2021-02-19
+  spec = builtins.fromJSON (builtins.readFile ./nixpkgs.json);
+  nixpkgs = fetchTarball {
+    url = "https://github.com/${spec.owner}/${spec.repo}/archive/${spec.rev}.tar.gz";
+    sha256 = spec.sha256;
+  };
+in
+import nixpkgs

examples/toolchains/cc_with_deps/shell.nix

@@ -1,6 +1,3 @@
-{ pkgs ? import (builtins.fetchTarball {
-  url = "https://github.com/NixOS/nixpkgs/archive/21.11.tar.gz";
-  sha256 = "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02";
-}) { } }:
+{ pkgs ? import ./nixpkgs.nix { } }:
 
 pkgs.mkShell { nativeBuildInputs = [ pkgs.bazel_4 ]; }

examples/toolchains/go/nixpkgs.json

@@ -0,0 +1,7 @@
+{
+  "owner": "NixOS",
+  "repo": "nixpkgs",
+  "branch": "21.11",
+  "rev": "506445d88e183bce80e47fc612c710eb592045ed",
+  "sha256": "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02"
+}

examples/toolchains/go/nixpkgs.nix

@@ -0,0 +1,9 @@
+let
+  # nixpkgs-unstable as of 2021-02-19
+  spec = builtins.fromJSON (builtins.readFile ./nixpkgs.json);
+  nixpkgs = fetchTarball {
+    url = "https://github.com/${spec.owner}/${spec.repo}/archive/${spec.rev}.tar.gz";
+    sha256 = spec.sha256;
+  };
+in
+import nixpkgs

examples/toolchains/go/shell.nix

@@ -1,6 +1,3 @@
-{ pkgs ? import (builtins.fetchTarball {
-  url = "https://github.com/NixOS/nixpkgs/archive/21.11.tar.gz";
-  sha256 = "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02";
-}) { } }:
+{ pkgs ? import ./nixpkgs.nix { } }:
 
 pkgs.mkShell { nativeBuildInputs = [ pkgs.bazel_4 ]; }

examples/toolchains/python/nixpkgs.json

@@ -0,0 +1,7 @@
+{
+  "owner": "NixOS",
+  "repo": "nixpkgs",
+  "branch": "21.11",
+  "rev": "506445d88e183bce80e47fc612c710eb592045ed",
+  "sha256": "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02"
+}

examples/toolchains/python/nixpkgs.nix

@@ -0,0 +1,9 @@
+let
+  # nixpkgs-unstable as of 2021-02-19
+  spec = builtins.fromJSON (builtins.readFile ./nixpkgs.json);
+  nixpkgs = fetchTarball {
+    url = "https://github.com/${spec.owner}/${spec.repo}/archive/${spec.rev}.tar.gz";
+    sha256 = spec.sha256;
+  };
+in
+import nixpkgs

examples/toolchains/python/shell.nix

@@ -1,6 +1,3 @@
-{ pkgs ? import (builtins.fetchTarball {
-  url = "https://github.com/NixOS/nixpkgs/archive/21.11.tar.gz";
-  sha256 = "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02";
-}) { } }:
+{ pkgs ? import ./nixpkgs.nix { } }:
 
 pkgs.mkShell { nativeBuildInputs = [ pkgs.bazel_4 ]; }

examples/toolchains/rust/nixpkgs.json

@@ -0,0 +1,7 @@
+{
+  "owner": "NixOS",
+  "repo": "nixpkgs",
+  "branch": "21.11",
+  "rev": "506445d88e183bce80e47fc612c710eb592045ed",
+  "sha256": "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02"
+}

examples/toolchains/rust/nixpkgs.nix

@@ -0,0 +1,9 @@
+let
+  # nixpkgs-unstable as of 2021-02-19
+  spec = builtins.fromJSON (builtins.readFile ./nixpkgs.json);
+  nixpkgs = fetchTarball {
+    url = "https://github.com/${spec.owner}/${spec.repo}/archive/${spec.rev}.tar.gz";
+    sha256 = spec.sha256;
+  };
+in
+import nixpkgs

examples/toolchains/rust/shell.nix

@@ -1,6 +1,3 @@
-{ pkgs ? import (builtins.fetchTarball {
-  url = "https://github.com/NixOS/nixpkgs/archive/21.11.tar.gz";
-  sha256 = "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02";
-}) { } }:
+{ pkgs ? import ./nixpkgs.nix { } }:
 
 pkgs.mkShell { nativeBuildInputs = [ pkgs.bazel_4 ]; }