Pipelines and arrows

2026-08-09 ocaml yocaml

The central type in YOCaml is ('a, 'b) Task.t: a task turning a value of type 'a into a value of type 'b while accumulating a set of dependencies.

Compose, don't configure

Building a page means chaining tasks together:

Pipeline.read_file_with_metadata (module Metadata.Article) source
>>> Yocaml_markdown.content_to_html ()
>>> Pipeline.chain_templates ~templates

Every >>> composes two tasks. The interesting part is implicit: reading a file adds that file to the dependency set, applying a template adds the template. When YOCaml decides whether a target needs rebuilding, it compares the target against that set — which nobody had to declare by hand.

The generator depends on itself

The detail I find elegant: track_file lets you declare the generator binary as a dependency of every page. Change the OCaml code that emits the HTML and the entire site is invalidated. That is exactly the behaviour you want, and it costs one line.

Validated front matter

The YAML header of an article is not a bag of strings. It goes through a module with a validate function that either builds a typed value or fails. In practice that means an article dated 2026-13-45 does not render a slightly wrong page — it breaks the build immediately, with a message pointing at the offending file.