shells/guile: init devshell

This is not a good devshell yet with incomplete toolset but it'll get
there.

shells/default.nix

@@ -5,6 +5,8 @@ with pkgs; {
   gnu = callPackage ./gnu.nix { };
   gnome = callPackage ./gnome.nix { };
   nix = callPackage ./nix.nix { };
+  guile = callPackage ./guile.nix { };
+  guile3 = callPackage ./guile.nix { guile = guile_3_0; };
   hugo = callPackage ./hugo.nix { };
   rust = callPackage ./rust.nix { };
   tic-80 = callPackage ./tic-80.nix { };

shells/guile.nix

@@ -0,0 +1,26 @@
+{ mkShell
+, lib
+, guile
+, guile-hall
+}:
+
+let
+  guileVersion = lib.versions.majorMinor guile.version;
+in
+mkShell {
+  inherit guileVersion;
+  packages = [
+    guile
+    guile-hall
+  ];
+
+  # This is already properly exported through setup hooks but to make
+  # intentions clearer.
+  shellHook = ''
+    export GUILE_LOAD_PATH GUILE_LOAD_COMPILED_PATH
+
+    if test $guileVersion == "3.0"; then
+      export GUILE_EXTENSIONS_PATH
+    fi
+  '';
+}