POC of overlays approach to customization

default.nix

@@ -1,4 +1,12 @@
-{ sources ? import ./nix/sources.nix
+let 
+  overlay = self: super: {
+    nixpkgs = super.nixpkgs-renamed;
+    __fetchers = super.__fetchers // {
+      fetchgit = {url, sha256}@attrs: (import self.nixpkgs {}).fetchgit attrs;
+    };
+  };
+in
+{ sources ? import ./nix/sources.nix { overlays = [overlay]; }
 , pkgs ? import ./nix { inherit sources; }
 }:
 

nix/default.nix

@@ -1,13 +1,8 @@
-{ sources ? import ./sources.nix }:
+{ sources ? import ./sources.nix {} }:
 import sources.nixpkgs
   { overlays =
-    [ (_: pkgs:
-        { inherit sources; }
-      )
-      (_: pkgs:
-        { termtosvg = pkgs.callPackage ./termtosvg.nix {}; }
-      )
-
+    [ (_: _: { inherit sources; })
+      (_: super: { termtosvg = super.callPackage ./termtosvg.nix {}; })
     ];
     config = {};
   }

nix/sources.json

@@ -1,13 +1,13 @@
 {
-    "nixpkgs": {
+    "nixpkgs-renamed": {
         "branch": "nixos-19.03",
         "description": "Nixpkgs/NixOS branches that track the Nixpkgs/NixOS channels",
         "homepage": "https://github.com/NixOS/nixpkgs",
         "owner": "NixOS",
         "repo": "nixpkgs-channels",
         "rev": "5b0b58685d68b9c9c12132f7b392c9dcdecc5931",
         "sha256": "1fy1ghl79p3w1vg0hnakvrb1lqic4mjrwmylcgqa5bd4ilfrirzf",
-        "type": "tarball",
+        "type": "builtin-tarball",
         "url": "https://github.com/NixOS/nixpkgs-channels/archive/5b0b58685d68b9c9c12132f7b392c9dcdecc5931.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
@@ -19,8 +19,7 @@
         "repo": "termtosvg",
         "rev": "b97cc0132073111cec37f64c95539e869202ff99",
         "sha256": "1c86622sda98zm3q3dzwmm37i09yca661kn860svl191nqbh2l68",
-        "type": "tarball",
-        "url": "https://github.com/nbedos/termtosvg/archive/b97cc0132073111cec37f64c95539e869202ff99.tar.gz",
-        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+        "type": "fetchgit",
+        "url": "https://github.com/nbedos/termtosvg.git"
     }
 }

nix/sources.nix

@@ -1,93 +1,75 @@
 # 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;
+{ overlays ? [] }:
+let 
+  # borrowed from nixpkgs
+  extends = f: rattrs: self: let super = rattrs self; in super // f self super;
+  fix = f: let x = f x; in x;
+  functionArgs = f: f.__functionArgs or (builtins.functionArgs f);
+  flip = f: a: b: f b a;
+  callFunctionWith = autoArgs: f: args:
+    let auto = builtins.intersectAttrs (functionArgs f) autoArgs;
+    in f (auto // args);
+  mapAttrs = builtins.mapAttrs or
+    (f: set: with builtins;
+      listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set)));
 
   hasNixpkgsPath = (builtins.tryEval <nixpkgs>).success;
   hasThisAsNixpkgsPath =
     (builtins.tryEval <nixpkgs>).success && <nixpkgs> == ./.;
 
-  sources = builtins.fromJSON (builtins.readFile ./sources.json);
+  niv-overlay = self: super: {
+    nixpkgs = 
+      if hasNixpkgsPath && !hasThisAsNixpkgsPath
+      then <nixpkgs>
+      else super.nixpkgs;
 
-  mapAttrs = builtins.mapAttrs or
-    (f: set: with builtins;
-      listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set)));
+    __fetchers = {
+      # A wrapper around pkgs.fetchzip that has inspectable arguments,
+      # annoyingly this means we have to specify them
+      tarball = { url, sha256 }@attrs: (import self.nixpkgs {}).fetchzip attrs;
+      # fetchTarball version that is compatible between all the versions of Nix
+      builtin-tarball = { url, sha256 }@attrs:
+        let inherit (builtins) lessThan nixVersion fetchTarball;
+        in
+          if lessThan nixVersion "1.12" 
+          then fetchTarball { inherit url; }
+          else fetchTarball attrs;
+      # A wrapper around pkgs.fetchurl that has inspectable arguments,
+      # annoyingly this means we have to specify them
+      file = { url, sha256 }@attrs: (import self.nixpkgs {}).fetchurl attrs;
+      # fetchurl version that is compatible between all the versions of Nix
+      builtin-url = { url, sha256 }@attrs:
+        let inherit (builtins) lessThan nixVersion fetchurl;
+        in
+          if lessThan nixVersion "1.12" 
+          then fetchurl { inherit url; }
+          else fetchurl attrs;
+    };
+  };
 
-  # 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);
+  sources = builtins.fromJSON (builtins.readFile ./sources.json);
 
-  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
+  sources-overlay = self: super:
+    let getFetcher = spec:
+      let fetcherName =
+        if builtins.hasAttr "type" spec
+        then builtins.getAttr "type" spec
+        else "builtin-tarball";
+      in builtins.getAttr fetcherName (self.__fetchers);
+    in mapAttrs (_: spec:
+      # NOTE: spec must _not_ have an "outPath" attribute
+      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;
+  toFix = builtins.foldl' (flip extends) (self: {}) ([ niv-overlay sources-overlay ] ++ overlays);
+in
+  fix toFix

script/test

@@ -1,6 +1,5 @@
 #!/usr/bin/env nix-shell
 #!nix-shell -i bash
-#!nix-shell -I nixpkgs=./nix
 #!nix-shell -p nix
 #!nix-shell --pure
 #!nix-shell --keep SSL_CERT_FILE