Factor out building Agda HTML from building the blog

This saves a lot of time since typechecking Agda is slow.

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

lib.nix

@@ -5,18 +5,42 @@ with lib;
 
 let
   protocol = ssl: if ssl then "https://" else "http://";
-  gems = bundlerEnv {
+  gems = settings: bundlerEnv {
     inherit ruby;
 
     name = "blog-static-flake";
-    gemfile = "${blog-source}/Gemfile";
-    lockfile = "${blog-source}/Gemfile.lock";
+    gemfile = "${settings.src}/Gemfile";
+    lockfile = "${settings.src}/Gemfile.lock";
     gemset = ./gemset.nix;
   };
 
+  # --- Building Agda HTML ---
+  agdaEnv = agda.withPackages [ agdaPackages.standard-library ];
+  agdaHtml = settings:
+    let
+      # Create content-addresed versions of the scripts in the Nix store to
+      # avoid throwing off memoization for unrelated changes.
+      agdaBuildScript = builtins.toFile "build-agda-html.rb" (builtins.readFile "${settings.src}/build-agda-html.rb");
+      submoduleDataFile = builtins.toFile "submodules.json" (builtins.readFile "${settings.src}/data/submodules.json");
+      codeRoot = pkgs.lib.cleanSource "${settings.src}/code";
+      agdaCommand = pkgs.lib.escapeShellArg "agda -l standard-library -i . ";
+    in
+      stdenv.mkDerivation {
+        name = "blog-static-agda-html";
+        version = settings.src.rev or "dirty";
+        buildInputs = [ ruby agdaEnv ];
+        builder = builtins.toFile "builder.sh" "
+        source $stdenv/setup
+        mkdir -p code $out
+        cp -r ${codeRoot}/* code/
+        chmod -R u+w code
+        ruby ${agdaBuildScript} --data-file=${submoduleDataFile} --target-dir=$out ${agdaCommand}
+        ";
+      };
+
   website = settings: stdenv.mkDerivation {
     inherit (settings) src ssl host;
-    inherit resume gems ruby;
+    inherit resume ruby;
     name = "blog-static";
     version = settings.src.rev or "dirty";
     publicPath = settings.path;
@@ -26,8 +50,10 @@ let
     );
     builder = ./build/builder.sh;
     webFiles = web-files;
+    gems = gems settings;
+    agdaHtml = agdaHtml settings;
     buildInputs = [
-      hugo ruby stork (agda.withPackages [ agdaPackages.standard-library ])
+      hugo ruby stork agdaEnv
     ];
   };
 in