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 fantasticuseR! 2025 conference, but, as promised, here is the third in a seriesof blog posts highlighting recent improvements to thefutureverse 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% offoreach will behave as if %dofuture% was used instead. This canbe 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 itvia doFuture::registerDoFuture("%dofuture%"), results in:

• Messages, warnings, and other types of conditions generated inparallel are relayed as-is regardless of parallel backend. This isalso 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. Incontrast, when using doFuture, it just works. You can also usecapture.output() and withCallingHandlers() as you would do whenyou run things sequentially

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

• If there are issues with parallel workers (e.g. they crash), moreinformative error messages will be given, which helps totroubleshoot the underlying problem. Futureverse may also attemptto relaunch crashed parallel workers so that you do not have toshut everything done and restart from scratch, which might be thecase otherwise

• Global variables and packages needed in the parallel tasks areidentified 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 viaFutureverse, also when %dopar% is used and the developer forgotto 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%, thenyou 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 yourfunction returns, the temporarily registered foreach adapter isundone. Easy!

When to use?

If you find that some foreach code that you cannot modify, orpackage that uses it, fails, gives hard-to-troubleshoot errors, ordoes 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 anew Futureverse feature that I’m very excited about. But more aboutthat later …

May the future be with you!

Henrik