foreach: Making All %dopar% Behave Like %dofuture% Everywhere

The future package celebrates ten years on CRAN as of June 19, 2025. I got a bit stalled over the holidays and going to the fantastic useR! 2025 conference, but, as promised, here is the third in a series of blog posts highlighting recent improvements to the futureverse ecosystem.

doFuture 1.10.0 is on CRAN as of May 2025 followed by two bug-fix releases in June and July. It comes with several improvements, but here I would like to focus on two new features:

1. registerDoFuture("%dofuture%"), and

2. with(doFuture::registerDoFuture("%dofuture%"), local = TRUE)

Turn all %dopar% into %dofuture%

By registering

doFuture::registerDoFuture("%dofuture%")

all existing code in scripts and packages that uses %dopar% of foreach will behave as if %dofuture% was used instead. This can be a step up from

doFuture::registerDoFuture()

which indeed tells %dopar% to parallelize via the future framework, but %dofuture% is even better – as I outline in my blog post ‘%dofuture% – a Better foreach() Parallelization Operator than %dopar%.’ For example, using an explicit %dofuture%, or forcing it via doFuture::registerDoFuture("%dofuture%"), results in:

• Messages, warnings, and other types of conditions generated in parallel are relayed as-is regardless of parallel backend. This is also true for standard output produced by print(), cat(), str(), etc. When using foreach without Futureverse (e.g. doParallel), such output is lost to the void. In contrast, when using doFuture, it just works. You can also use capture.output() and withCallingHandlers() as you would do when you run things sequentially

• Errors generated in parallel are relayed as-is without them being wrapped up in other error objects. With %dofuture% they behave as you would expect, it becomes easier to troubleshoot errors, and errors can be handled by tryCatch(). This can be particularly useful if you use code that uses %dopar% but that does not handle errors

• If there are issues with parallel workers (e.g. they crash), more informative error messages will be given, which helps to troubleshoot the underlying problem. Futureverse may also attempt to relaunch crashed parallel workers so that you do not have to shut everything done and restart from scratch, which might be the case otherwise

• Global variables and packages needed in the parallel tasks are identified automatically and consistently by Futureverse, regardless of which parallel backend is used

In additions, it is possible to make:

• Parallel random number generation (RNG) be done consistently via Futureverse, also when %dopar% is used and the developer forgot to use %dorng%

• Get near-live progress updates via the progressr package, also when %dopar% is used

For package developers

If you’re a developer and call another package that uses foreach() with %dopar% internally, but wish it was using %dofuture%, then you can add

with(doFuture::registerDoFuture("%dofuture%"), local = TRUE)

to the top of your function. This will temporarily make %dopar% behave like %dofuture% while your function is used. As soon as your function returns, the temporarily registered foreach adapter is undone. Easy!

When to use?

If you find that some foreach code that you cannot modify, or package that uses it, fails, gives hard-to-troubleshoot errors, or does not output expected messages and warnings, then try with

doFuture::registerDoFuture("%dofuture%")
# code or functions that call foreach(...) %dopar% { ... }

or

with(doFuture::registerDoFuture("%dofuture%"), {
  # code or functions that call foreach(...) %dopar% { ... }
})

It might solve your problems, or at least, help you narrow them down.

Also, this new doFuture feature lays part of the foundation for a new Futureverse feature that I’m very excited about. But more about that later …

May the future be with you!

Henrik