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Add a progressr-based progress bar for sampling.#1138

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josswright:progressr
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Add a progressr-based progress bar for sampling.#1138
josswright wants to merge 16 commits intostan-dev:masterfrom
josswright:progressr

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@josswright josswright commented Feb 13, 2026

Submission Checklist

  • Run unit tests
  • Declare copyright holder and agree to license (see below)

Summary

As discussed in #18 , this pull request adds a progress bar to CmdStanR's sample() function via the progressr framework.

This can be enabled via two new options to model$sample(...):

  • show_progress_bar - boolean - default FALSE
    Enables the progress bar.
  • suppress_iteration_messages - boolean - default FALSE if show_progress_bar is FALSE, and TRUE if show_progress_bar is TRUE.
    Causes any output lines reporting an iteration (such as Chain 1 Iteration 1 / 2000 (Warmup)) to be suppressed.

For the progress bar to be displayed, a progress bar handler must be created and registered by the end user. (See: https://progressr.futureverse.org/) For ease of use, a register_default_progress_handler() function has been added. Calling register_default_progress_handler() before a model$sample(...) call will spawn a default progress bar via the cli library.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):

Joss Wright, University of Oxford

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

josswright and others added 10 commits December 21, 2025 22:55
This uses the
[`progressr`](https://progressr.futureverse.org/index.html) framework to
enable a progress bar for sampling operations. By default, this replaces
standard iteration messages, but not other informative messages produced
during sampling.

From a user point of view, this adds two arguments to the `$sample()`
method:
- `show_progress_bar`: Default = FALSE

  If TRUE, registers a progress bar via `progressr` and signals an
  update for every output line that matches "Iteration:". The user is
  responsible for registering those progress updates with an appropriate
  handler.

- `suppress_iteration_messages`: Defaults to the value of
  show_progress_bar, but can also be set directly.

  If TRUE, disables display of output lines matching "Iteration:", while
  still causing the progress bar to update. This keeps all of Stan's
  other informative output, but just removes the superfluous iteration
  messages when using a progress bar.

I've tried to keep all additions to the code as non-intrusive as
possible.

One minor decision I made was to pass the value of `refresh` from the
`$sample()` method through to the `CmdStanProcs` class so that the
progress bar can report and update the number of steps in the bar to the
same 'scale' as the number of iterations. (Calling `$sample()` with
`iter_sampling=8000` and `refresh=100` will result in 80 'ticks' of the
progress bar, each increasing the progress by 100.)

By default, `progressr` doesn't register a handler to display the
progress bar, as this is the responsibility of the user. Multiple
packages can be used to display the progress bar. A default progress bar
can be registered using the [`cli`](https://cli.r-lib.org/) library in
this way:

```r
library(progressr)
library(cli)

handlers( global=TRUE )
handlers("cli")

options( cli.spinner = "moon",
         cli.progress_show_after = 0,
         cli.progress_clear = FALSE )

handlers( handler_cli(
            format = "{cli::pb_spin} Progress: |{cli::pb_bar}| {cli::pb_current}/{cli::pb_total} | {cli::pb_percent} | ETA: {cli::pb_eta}",
            clear = FALSE ))
```

A variety of alternative progress bar handlers are available, including
audible and notification-based handlers, and ones that interact with
RStudio's jobs pane:
<https://progressr.futureverse.org/articles/progressr-11-handlers.html>
In addition to updating the progress bar on stdout lines that match
"Iteration:", the CmdStanProcs object(s) now pass the current stdout
line to the progress bar as a message for _every_ line, even if the
number of completed iterations hasn't increased.
Added a `register_default_progress_handler()` function to `utils.R` that
creates a default progress bar for sampling operations and registers it
as global handler for progressr. Requires `cli` and `progressr`.
Was accidentally passing `refresh` to each informational progress bar
update, rather than the calculated `progress_amount`. This caused each
message line to update the progress bar, even if it didn't match an
iteration message.
Moved code to close the progress bar to after the `check_finished()`
function.
The nature of CmdStan's output makes pulling appropriate refresh and
update values for the progress bar slightly tricky. In the original
verison of the code, this led to the bar occasionally updating past the
point where it was full, or terminating early due to an incorrectly
calculated number of steps.

This commit reworks the calculations of updates and refresh values for
the progress bar, as well as adding some extra conditions to avoid the
bar potentially crashing in odd scenarios.
New arguments for `iter_warmup` and `iter_sampling` were not being
assigned a default value if not specified in sampling statement.
@VisruthSK VisruthSK linked an issue Feb 15, 2026 that may be closed by this pull request
@VisruthSK
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Looks like some tests are failing because the lower bound for iter_sampling should be 0 instead of 1.

@josswright
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Looks like some tests are failing because the lower bound for iter_sampling should be 0 instead of 1.

Great. Thanks for this. (First pull request for me on a project like this, so I was a little unfamiliar with the process and running the unit tests.)

I've fixed the initial errors, and a couple of other warnings. Now running the tests and documentation generation commands locally before I push an update.

josswright and others added 2 commits February 15, 2026 15:46
- `iter_{warmup,sampling}` arguments to MCMCProcs are allowed to be 0.
- Documentation for `register_default_progress_handler` added to
  repository.
- Ensured that calls to `progressr` functions are appropriately
  prefixed.
@josswright josswright marked this pull request as draft February 15, 2026 18:37
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codecov-commenter commented Feb 15, 2026

Codecov Report

❌ Patch coverage is 40.29851% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.92%. Comparing base (453084b) to head (6ea01d9).

Files with missing lines Patch % Lines
R/run.R 50.00% 19 Missing ⚠️
R/utils.R 0.00% 14 Missing ⚠️
R/model.R 53.33% 7 Missing ⚠️
Additional details and impacted files
@@            Coverage Diff             @@
##           master    #1138      +/-   ##
==========================================
- Coverage   87.49%   86.92%   -0.58%     
==========================================
  Files          14       14              
  Lines        5983     6087     +104     
==========================================
+ Hits         5235     5291      +56     
- Misses        748      796      +48     

☔ View full report in Codecov by Sentry.
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josswright and others added 4 commits February 15, 2026 19:13
Added documentation for `show_progress_bar` and
`suppress_iteration_messages` in `sample()` function.

Changed `require()` calls for `progressr` and `cli` to suggested
`requireNamespace()` alternatives.
@josswright josswright marked this pull request as ready for review February 16, 2026 17:44
@josswright
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Thanks for the help with the unit tests.

In the spirit of peer review, some notes and thoughts:

  • I'm not entirely happy with CmdStan's default iter_warmup and iter_sampling values being hard-coded into MCMCProcs default call rather than derived somehow. I imagine that the default values are fairly fixed, but I would have liked not to rely on that. Every route I could see to getting them was clunky and brought in unnecessary code.

  • As mentioned above, I toyed with the idea of setting refresh to 1 by default when showing the progress bar with iteration messages suppressed. In the end that seemed to introduce yet another case of a parameter depending on another parameter, as well as a potential source of fragility. I'll note, though, that refresh can safely be set to 1 for a nice, smooth progress bar without any noticeable overhead or slowdown that I saw in testing.

  • The code to derive the number of steps for the progress bar is more complicated than I originally imagined because of how CmdStan reports iterations.

    At one stage, when the number of steps was incorrectly calculated as too low, the progress bar was crashing, which also killed the sampling run. I've now ensured that the progress bar automatically closes when it reaches its finished number of steps, which should avoid crashes and just print a warning in the case of some output oddity causing more update steps to be signalled.

    I've tested as extensively as I can, but I live in fear of this killing someone's multi-day sampling run.

@jgabry
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jgabry commented Feb 18, 2026

Awesome, thank you for the PR! I think I have the flu right now, but I will take a look when I'm feeling better and caught up on work.

@josswright
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Awesome, thank you for the PR! I think I have the flu right now, but I will take a look when I'm feeling better and caught up on work.

No rush on my side! Hope you feel better soon.

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Progress bar

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