Pin nixpkgs and add CI

ci.nix

@@ -1 +1 @@
-import ./tests/default.nix
+import ./tests/default.nix { pkgs = import ./nix {}; }

nix/default.nix

@@ -0,0 +1,9 @@
+{ sources ? import ./sources.nix }:
+let
+  config = {};
+  overlays = [(super: self: {
+    inherit (import sources.niv {}) niv;
+  })];
+  pkgs = import sources.nixpkgs { inherit overlays config; };
+in
+  pkgs

nix/sources.json

@@ -0,0 +1,25 @@
+{
+    "nixpkgs": {
+        "url": "https://github.com/NixOS/nixpkgs-channels/archive/312a059bef8b29b4db4e73dc02ff441cab7bb26d.tar.gz",
+        "owner": "NixOS",
+        "branch": "nixos-19.03",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz",
+        "repo": "nixpkgs-channels",
+        "type": "tarball",
+        "sha256": "1j52yvkhw1inp6ilpqy81xv1bbwgwqjn0v9647whampkqgn6dxhk",
+        "description": "Nixpkgs/NixOS branches that track the Nixpkgs/NixOS channels",
+        "rev": "312a059bef8b29b4db4e73dc02ff441cab7bb26d"
+    },
+    "niv": {
+        "homepage": "https://github.com/nmattia/niv",
+        "url": "https://github.com/nmattia/niv/archive/5d9e3a5f7d51765f0369a4682770ec57d863f19f.tar.gz",
+        "owner": "nmattia",
+        "branch": "master",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz",
+        "repo": "niv",
+        "type": "tarball",
+        "sha256": "0x7d2rb89h0h7g8sjsgax6ncvf2wwbmxkgvlfi53d00kxj6kfzba",
+        "description": "Easy dependency management for Nix projects",
+        "rev": "5d9e3a5f7d51765f0369a4682770ec57d863f19f"
+    }
+}

nix/sources.nix

@@ -0,0 +1,67 @@
+# 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.
+    '';
+
+  builtins_fetchTarball =
+      # fetchTarball version that is compatible between all the versions of
+      # Nix
+      { url, sha256 }@attrs:
+      let
+        inherit (builtins) lessThan nixVersion fetchTarball;
+      in
+        if lessThan nixVersion "1.12" then
+          fetchTarball { inherit url; }
+        else
+          fetchTarball 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)));
+
+  getFetcher = spec:
+    let fetcherName =
+      if builtins.hasAttr "type" spec
+      then builtins.getAttr "type" spec
+      else "tarball";
+    in builtins.getAttr fetcherName {
+      "tarball" = pkgs.fetchzip;
+      "file" = pkgs.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 = getFetcher spec { inherit (spec) url sha256; } ; }
+    else spec
+  ) sources

shell.nix

@@ -0,0 +1,5 @@
+{ pkgs ? import ./nix {}}:
+pkgs.mkShell {
+  name = "dev-shell";
+  buildInputs = [ pkgs.niv ];
+}