Pin shell channel with niv

nix/sources.json

@@ -0,0 +1,14 @@
+{
+    "nixpkgs": {
+        "branch": "nixos-unstable",
+        "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": "7c399a4ee080f33cc500a3fda33af6fccfd617bd",
+        "sha256": "0vqljvz5yrc8i3nj3d5xiiv475yydscckfc9z0hpran9q2rh4md1",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs-channels/archive/7c399a4ee080f33cc500a3fda33af6fccfd617bd.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    }
+}

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); }

shell.nix

@@ -1,11 +1,11 @@
-{ pkgs ? import (fetchTarball { url = "channel:nixpkgs-unstable"; }) {
+let
+  sources = import ./nix/sources.nix;
+  pkgs = import sources.nixpkgs {
     overlays = [
       (import ./overlay.nix)
     ];
-  }
-}:
-let
-  tools = pkgs.callPackage ./tools {};
+  };
+  tools = pkgs.callPackage ./tools { };
 in
 pkgs.mkShell {
   buildInputs = [
@@ -13,5 +13,6 @@ pkgs.mkShell {
     tools.release
     pkgs.nixpkgs-fmt
     pkgs.poetry
+    pkgs.niv
   ];
 }