updated draft on achille 2.0

content/assets/theme.css

@@ -111,12 +111,19 @@ hr {
   padding: 0;
   margin: 2em 0 0;
 }
-#pidx li {line-height: 1.6em}
+#pidx li {
+  line-height: 1.4em; display: flex; 
+  margin: 1em 0;
+}
 #pidx li > span {
   font-family: monospace;
   font-size: 14px;
   margin-right: 1em;
   opacity: 0.7;
+  flex-shrink: 0;
+}
+#pidx li a {
+  box-shadow: none;
 }
 
 details summary {

content/posts/achille-smc.md

@@ -1,35 +1,106 @@
 ---
-title: Building my site with monoidal categories
+title: Generating incremental static site generators in Haskell using cartesian categories
 date: 2022-12-06
 draft: true
+toc: true
 ---
 
-Or how the right theoretical framework solved the last problem I had in the way
+A few days ago, I released the new version of [achille], a Haskell library
-of incremental generation for "free": reasoning about dependencies optimally.
+providing an EDSL for writing static site generators. This embedded language produces
+efficient, *incremental* and *parallel* static site generators, *for free*.
+
+[achille]: /projects/achille
+
+In this post, I will explain how [achille] is able to tranform this intuitive, "readable"
+syntax into an incremental static site generator:
+
+```haskell
+import Achille as A
+
+main :: IO ()
+main = achille $ task A.do
+  -- render every article in `posts/`
+  -- and gather all metadata
+  posts <-
+    match "posts/*.md" \src -> A.do
+      (meta, content) <- processPandocMeta src
+      writeFile (src -<.> ".html") (renderPost meta content)
+      meta
+
+  -- render index page with the 10 most recent articles
+  renderIndex (take 10 (sort posts))
+```
+
+
+Importantly, I want to emphasize that *you* --- the library user --- neither
+have to care about or understand the internals of [achille] in order to use it.
+You are free to ignore this post and directly go through the [user
+manual][manual] to get started!
+
+[manual]: /projects/achille/
+
+This post is just there to document how the right theoretical framework was key
+in providing a good user interface that preserves all the desired properties.
 
 ---
 
-A while back I made [achille](/projects/achille), a library for building
+## Foreword
-incremental static site generators in Haskell. I'm not gonna delve into *why*
-for long, if you want the full motivation you can read the details in the
-(outdated) [documentation](/projects/achille/1-motivation.html).
 
-The point was:
+The original postulate is that *static sites are good*. Of course not for every
+use case, but for single-user, small-scale websites, it is a very practical way
+of managing content. Very easy to edit offline, very easy to deploy. All in all
+very nice.
 
-- static sites are good, therefore one wants to use static site generators.
+There are lots of static site generators readily available. However each and
-- the way to build their site becomes quite intricate and difficult to express
+every one of them has a very specific idea of how you *should* manage your
-  with existing static site generators.
+content. For simple websites --- i.e weblogs --- they are great, but as soon as
-- thus one ends up making their own custom generator suited for the task.
+you want to heavily customize the building process of your site, require more
+fancy transformations, and thus step outside of the supported feature set of
+your site generator of choice, you're in for a lot of trouble.
 
-Making your own static site generator is not very hard, but making it
+For this reason, many people end up not using existing static site generators,
-*incremental* is tedious and requires some thinking.
+and instead prefer to write their own. Depending on the language you use, it is
+fairly straightforward to write a little static site generator doing everything
+you want. Sadly, making it *incremental* or *parallel* is another issue, and way
+trickier.
 
-That's the niche that [Hakyll](https://jaspervdj.be/hakyll/) tries to fill: an
+That's precisely the niche that [Hakyll] and
-embedded DSL in Haskell to specify your build rules, and compile them into a
+[achille] try to fill: use an embedded DSL in Haskell to specify your *custom* build
-full-fletched **incremental** static site generator. Some kind of static site
+rules, and compile them all into a full-fletched **incremental** static site
-generator *generator*.
+generator executable. Some kind of static site generator *generator*.
 
-## achille, as it used to be
+[Hakyll]: https://jaspervdj.be/hakyll/
+
+## Reasoning about static site generators
+
+Let's look at what a typical site generator does. A good way to visualize it
+is with a flow diagram, where *boxes* are "build rules". Boxes have
+distinguished inputs and outputs, and dependencies between the build rules are
+represented by wires going from outputs of boxes to inputs of other boxes.
+
+The static site generator corresponding to the Haskell code above could be
+represented as the following diagram:
+
+...
+
+Build rules are clearly identified, and we see that in order to render the `index.html`
+page, we need to wait for the `renderPosts` rule to finish rendering each
+article to HTML and return the metadata of every one of them.
+
+Notice how some wires are **continuous** **black** lines, and some other wires are
+faded **dotted** lines. The **dotted lines** represent **side effects** of the
+generator.
+
+- files that are read from the file system, like all the markdown files in
+    `posts/`.
+- files that are written to the filesystem, like the HTML output of every
+    article, or the `index.html` file.
+
+The first insight is to realize that the build system *shouldn't care about side
+effects*. Its *only* role is to know whether build rules *should be executed*,
+and how intermediate values get passed around.
+
+### The `Recipe m` abstraction
 
 I had my gripes with Hakyll, and was looking for a simpler, more general way to
 express build rules. I came up with the `Recipe` abstraction: