k7: Keeping CLI Animations in Sync with Your Code

I have always been a fan of asciinema and of Charm's VHS. asciinema records a terminal while you use it, and VHS types a script of commands into a terminal and films the result. I wanted their animations in the READMEs of my projects, but I always forgot the syntax of VHS's tape files, and I always mistyped a command halfway through an asciinema recording (and then started again). I also always forgot to re-run those recordings when my API changed, and they got out of sync quickly. Fortunately, most of my projects already check their command-line output with cram tests, transcripts of shell commands and of what they print, which dune, OCaml's build system, runs as tests. Making those tests produce animated images was a short stretch, and then, why stop there? The result is k7 (how the French write cassette, if you say it out loud), a command-line tool that records any program, whatever its language, as an SVG image animated with CSS alone. Here is ls, and sl, which punishes you for…