chore(dev): re-configure Nix environment

- All important Nix expressions are now under `./nix/default.nix`.
- Switched from telos.nix to vanilla nixpkgs (branch `release-22.11`).

nix/default.nix

@@ -1,19 +1,57 @@
-{ compiler ? "ghc90"
-, ...
+{ ...
 }:
 
 let
   ## Import sources:
   sources = import ./sources.nix;
 
-  ## Import telosnix:
-  telosnix = import sources.telosnix { };
-
   ## Import nixpkgs:
-  pkgs = import telosnix.pkgs-sources.unstable { };
+  pkgs = import sources.nixpkgs { };
+
+  ## Define R package dependencies:
+  rDeps = with pkgs.rPackages; {
+    ## Dependencies for our package:
+    pkg = [
+      mvtnorm
+      tseries
+    ];
+
+    ## Dependencies for development:
+    dev = [
+      devtools
+      roxygen2
+      spelling
+      testthat
+    ];
+  };
+
+  ## Setup R environment:
+  r = pkgs.rWrapper.override {
+    packages = rDeps.pkg ++ rDeps.dev;
+  };
+
+  ## Setup development shell:
+  shell = pkgs.mkShell
+    {
+      buildInputs = [
+        ## Fancy stuff:
+        pkgs.figlet
+        pkgs.lolcat
+
+        ## TeX stuff:
+        pkgs.texlive.combined.scheme-full
+
+        ## R:
+        r
+      ];
+
+      shellHook = ''
+        figlet -w 999 "mgarchBEKK DEV SHELL" | lolcat -S 42
+      '';
+    };
 in
 {
-  sources = sources;
-  telosnix = telosnix;
   pkgs = pkgs;
+  r = r;
+  shell = shell;
 }

nix/sources.json

@@ -1,14 +1,14 @@
 {
-    "telosnix": {
-        "branch": "v0.0.4",
-        "description": null,
-        "homepage": null,
-        "owner": "telostat",
-        "repo": "telos.nix",
-        "rev": "4d45ee0f57fb6979781b726cdf87eb68f4ef672d",
-        "sha256": "0my9pmbvyfpj3n747zmwhl6fk8mhnmirl18zmlpy29g1pcd891k2",
+    "nixpkgs": {
+        "branch": "release-22.11",
+        "description": "Nix Packages collection",
+        "homepage": "",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "52e3e80afff4b16ccb7c52e9f0f5220552f03d04",
+        "sha256": "0x0pc4qm0adsdbr8n20ghl7byq9n0bvj9fjiiqzp9839cippp9hn",
         "type": "tarball",
-        "url": "https://github.com/telostat/telos.nix/archive/4d45ee0f57fb6979781b726cdf87eb68f4ef672d.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/52e3e80afff4b16ccb7c52e9f0f5220552f03d04.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }

shell.nix

@@ -3,34 +3,5 @@
 let
   ## Import this codebase's Nix helper set:
   nix = import ./nix { };
-
-  ## Get packages:
-  pkgs = nix.pkgs;
-
-  ## R environment:
-  r = pkgs.rWrapper.override {
-    packages = with pkgs.rPackages; [
-      devtools
-      mvtnorm
-      roxygen2
-      spelling
-      testthat
-      tseries
-    ];
-  };
 in
-pkgs.mkShell {
-  buildInputs = [
-    ## Fancy stuff:
-    pkgs.figlet
-    pkgs.lolcat
-
-    ## Development:
-    pkgs.texlive.combined.scheme-full
-    r
-  ];
-
-  shellHook = ''
-    figlet -w 999 "mgarchBEKK DEV SHELL" | lolcat -S 42
-  '';
-}
+nix.shell