Nix tweaks (#252)

* Pins nixpkgs

* Adds required texlive packages

* Factors texlive out of mkShell

* Sources Python packages from Nix

pinned.nix

@@ -0,0 +1,8 @@
+# This commit corresponds to a recent (as of 2 August 2020) pin of the 20.03
+# nixpkgs branch.
+import (
+  builtins.fetchTarball {
+    url = "https://github.com/nixos/nixpkgs/archive/7dc4385dc7b5b2c0dbfecd774cebbc87ac05c061.tar.gz";
+    sha256 = "10xr87ilxypz8fya6q42vsk5rmavv1bjrxhkznr9fvn514am58xb";
+  }
+)

shell.nix

@@ -1,67 +1,106 @@
-{ pkgs ? import <nixpkgs> {} }: with pkgs;
+{ pkgs ? import ./pinned.nix {} }:
 
-mkShell {
-  FONTCONFIG_FILE = makeFontsConf { fontDirectories = [ inconsolata-lgc libertine libertinus]; };
-  buildInputs = [
-    (texlive.combine {
-        inherit (texlive)
-        bookcover
-        textpos
-        fgruler
-        tcolorbox
-        fvextra
-        framed
-        newtx
-        nowidow
-        emptypage
-        wrapfig
-        subfigure
-        adjustbox
-        collectbox
-        tikz-cd
-        imakeidx
-        idxlayout
-        titlesec
-        subfiles
-        lettrine
-        upquote
-        libertine
-        mweights
-        fontaxes
-        mdframed
-        needspace
-        xifthen
-        ifnextok
-        currfile
-        noindentafter
-        ifmtarg
-        scheme-medium
-        listings
-        minted
-        microtype
-        babel
-        todonotes
-        chngcntr
-        ifplatform
-        xstring
-        minifp
-        titlecaps
-        enumitem
-        environ
-        trimspaces
-        l3packages;
-      })
+let
+  #############################################################################
+  # LaTeX Environment
+  texliveEnv = pkgs.texlive.combine {
+    inherit (pkgs.texlive)
+      bookcover
+      textpos
+      fgruler
+      tcolorbox
+      fvextra
+      framed
+      newtx
+      nowidow
+      emptypage
+      wrapfig
+      subfigure
+      adjustbox
+      collectbox
+      tikz-cd
+      imakeidx
+      idxlayout
+      titlesec
+      subfiles
+      lettrine
+      upquote
+      libertine
+      mweights
+      fontaxes
+      mdframed
+      needspace
+      xifthen
+      ifnextok
+      currfile
+      noindentafter
+      ifmtarg
+      scheme-medium
+      listings
+      minted
+      microtype
+      babel
+      todonotes
+      chngcntr
+      ifplatform
+      xstring
+      minifp
+      titlecaps
+      enumitem
+      environ
+      trimspaces
+      l3packages
+      zref
+      catchfile
+      import
+      ;
+  };
+
+  #############################################################################
+  # Python Environment
+
+  # Pin the Python version and its associated package set in a single place.
+  python = pkgs.python38;
+  pythonPkgs = pkgs.python38Packages;
+
+  pygments-style-github = pythonPkgs.buildPythonPackage rec {
+    pname = "pygments-style-github";
+    version = "0.4";
+
+    doCheck = false; # Hopefully someone else has run the tests.
+
+    src = pythonPkgs.fetchPypi {
+      inherit pname version;
+      sha256 = "19zl8l5fyb8z3j8pdlm0zbgag669af66f8gn81ichm3x2hivvjhg";
+    };
+
+    # Anything depending on this derivation is probably also gonna want
+    # pygments to be available.
+    propagatedBuildInputs = with pythonPkgs; [ pygments ];
+  };
+
+  pythonEnv = python.withPackages (
+    pyPkgs: with pyPkgs; [
+      pygments
+      pygments-style-github
+    ]
+  );
+in
+
+pkgs.mkShell {
+  FONTCONFIG_FILE = pkgs.makeFontsConf {
+    fontDirectories = with pkgs; [ inconsolata-lgc libertine libertinus ];
+  };
+
+  buildInputs = with pkgs; [
+    # Misc. build tooling. 
     gnumake
-    which
     git
     python3Packages.virtualenv
+    which
+    # LaTeX Environment (with all associated libraries and packages).
+    texliveEnv
+    # Python Environment (with all associated libraries and packages).
+    pythonEnv
   ];
-  shellHook = ''
-    # set SOURCE_DATE_EPOCH so that we can use python wheels
-    SOURCE_DATE_EPOCH=$(date +%s)
-    virtualenv venv
-    venv/bin/pip install -v pygments
-    venv/bin/pip install -v pygments-style-github
-    source venv/bin/activate
-  '';
-}
+}