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Hand-writing tape scripts

tape-scripts.Rmd

record_demo() and friends cover the typical case — type a block of R code into a real REPL, get a GIF. When you outgrow that, the underlying charmbracelet/vhs tape syntax is more general than record_demo() exposes. This vignette walks through the primitives you’ll want, the corners that bite, and how to hand a tape file or string to vhsr_run_tape().

When to leave record_demo() behind

record_demo() assumes you want to type R into a freshly-spawned R REPL. Reach for hand-written tape when:

  • You want to record a shell session (git, npm, pip, …) rather than R code.
  • You need multi-shell setup before R starts — clearing the screen, exporting an environment variable, navigating into a project directory.
  • You want fine-grained timing: per-key Backspace corrections, arrow-key navigation, Wait /regex/ synchronisation against output.
  • You want multiple Screenshot directives in a single recording for a flipbook.

For everything else, record_demo() saves typing.

The primitives, R-flavoured

The full vhs tape reference lives at the charmbracelet/vhs README; this is a working subset.

Set: terminal configuration 

Set Shell    "bash"
Set FontSize 22
Set Width    1200
Set Height   600
Set Theme    "Catppuccin Mocha"
Set TypingSpeed 60ms

Set directives must come before any input (Type, Enter, …). record_demo() always emits them at the top of the tape.

Type: simulated keystrokes 

Type "echo Hello"

Type@<duration> overrides the global TypingSpeed for one line:

Type@30ms "this types at 30ms per char"

vhsR uses this for typing_speed_jitter.

Keys 

Enter
Enter 3            # press Enter three times
Backspace 5
Tab
Space
Up    / Down / Left / Right
Ctrl+C

Sleep and Wait 

Sleep 800ms
Sleep 2s

Wait /regex/ blocks until the on-screen content matches:

Type "long-running-command"
Enter
Wait /Done\.$/    # waits up to 15s by default

Useful when the next typed input would be too early — for instance, waiting for an R prompt to redraw before continuing.

Hide / Show 

Hide
Type "stuff that shouldn't appear in the recording"
Enter
Show

Hide does not pause execution. The shell still runs typed input; vhs just stops adding frames to the output. record_demo() uses this to hide the R --quiet --no-save (or arf, or radian) startup.

Source, Env, Screenshot 

Source "common-setup.tape"     # include another tape file
Env DEMO_VAR "true"            # env var for the spawned shell
Screenshot "frame.png"         # save a PNG of the current frame

Screenshot is what record_demo_screenshot() calls through to.

Gotchas vhsR works around for you

Three corners of vhs tape syntax that vhsR papers over internally; you’ll want to copy the pattern when writing tape by hand.

  1. Slashes in unquoted paths look like a Wait regex literal. vhs parses /something/ as a regex. So this fails:

    Output /tmp/demo.gif
Screenshot /tmp/frame.png

    Quote the paths:

    Output "/tmp/demo.gif"
Screenshot "/tmp/frame.png"

  2. Strings with double quotes inside Type need backtick fallback:

    Type "x <- \"hello\""    # broken — vhs doesn't handle the escape
Type `x <- "hello"`      # works — vhs treats backticks as quotes

    vhsR:::quote_for_tape() picks the right form per line.

  3. Screenshot is occasionally racy. vhs takes the screenshot the moment the directive runs in the tape, but the headless renderer sometimes hasn’t repainted yet. Leave a Sleep 200ms (or longer) immediately before each Screenshot to be safe.

Running a hand-written tape

Pass a file path:

vhsr_run_tape("scripts/my-demo.tape")

…or a multi-line string:

vhsr_run_tape('
Output "demo.gif"
Set Shell "bash"
Set FontSize 22
Type "echo Hello from a hand-written tape!"
Enter
Sleep 800ms
Type `ls -1 *.R | head -3`
Enter
Sleep 1s
')

Both code paths go through the same vhs binary as record_demo(), with the same vhsr_check() gate and the same path resolution (vhsr_vhs_path() etc.). Anything you can express in tape syntax, this function can run.

The example above produces:

Version pinning

vhsR’s vhsr_install() pins to a specific upstream vhs version (DEFAULT_VHS_VERSION in R/install.R). If you’ve written tape that relies on a newer vhs feature, install that version explicitly:

vhsr_install(versions = list(vhs = "0.12.0"))

…or point at a system-installed vhs by setting vhsR.vhs_path / VHSR_VHS. See ?vhsr_vhs_path for the resolution chain.

See also