# future.tests: An Introduction

The future package defines the Future API which consists of a small number of functions for writing R code that can be evaluated either sequentially or in parallel based on a single setting without having to change anything in the code. Parallelization can be done via one of many backends, e.g. via built-in multicore, multisession and cluster backends (based on the parallel package) or via third-party backends such as future.batchtools, future.callr and future.mirai. The design motto of the Future API is:

> Write once, run anywhere

In order for such code to work regardless of which future backend the end-user chooses, it is critical that the backend fully complies with the Future API Backend Specification. A future backend with a 100% compliance rate guarantees that the code will work equally well there as in sequential mode.

This R package - future.tests - provides a test suite for validation that a future backend complies with the Future API.

## Validate a Future Backend

All future backends implementing the Future API should validate that they conform to the Future API. This can be done using the future.tests package, which provides two APIs for running the tests. The tests can be performed either from within R or from outside of R from the command line making it easy to include them in package tests and in Continuous Integration (CI) pipelines.

### From Within R

    > results <- future.tests::check(plan = "multisession")
    > exit_code <- attr(results, "exit_code")
    > if (exit_code != 0) stop("One or more tests failed")

### From Outside R

    $ Rscript -e future.tests::check --test-plan="multisession"
    $ exit_code=$?
    $ [[ exit_code -eq 0 ]] || { >&2 echo "One or more tests failed"; exit 1; }