coro

Overview

coro implements coroutines for R, i.e. functions that can be suspended and resumed later on. There are two kinds:

Async functions, which make it straightforward to program concurrently
Generators for iterating over complex sequences

Supported features:

Suspending within loops and if/else branches
Suspending within tryCatch()
on.exit() expressions and stack-based cleanup such as provided by local_ functions in the withr package
Step-debugging and browser() within coroutines

Compatibility with:

Python iterators from the reticulate package
Async operations from the promises package
Parallel computations from the future package

Attach the package to follow the examples:

library(coro)

Async/await functions

Concurrent programming is made straightforward by async-await functions. Whenever you are waiting for a result that may take a while (downloading a file, computing a value in an external process), use await(). The argument to await() must return a promise from the promises package.

Concurrent code based on promises can quickly become hard to write and follow. In the following artificial example, we wait for a download to complete, then decide to launch a computation in an external process depending on a property of the downloaded data. We also handle some errors specifically.

my_async <- function() {
  async_download() %>%
    then(function(data) {
      if (ncol(data) > 10) {
        then(future::future(fib(30)), function(fib) {
          data / fib
        })
      } else {
        data
      }
    }, onRejected = function(err) {
      if (inherits(err, "download_error")) {
        NULL
      } else {
        stop(err)
      }
    })
}

Rewriting this function with async/await greatly simplifies the code:

my_async <- async(function() {
  data <- tryCatch(
    await(async_download()),
    download_error = function(err) NULL
  )

  if (is.null(data)) {
    return(NULL)
  }

  if (ncol(data) > 10) {
    fib <- await(future::future(fib(30)))
    data <- data /fib
  }

  data
})

Generators

Generators are based on a simple iteration protocol:

Iterators are functions.
They can be advanced by calling the function. The new value is returned.
An exhausted iterator returns the sentinel symbol exhausted.

The generator() function creates a generator factory which returns generator instances:

# Create a generator factory
generate_abc <- generator(function() {
  for (x in letters[1:3]) {
    yield(x)
  }
})

# Create a generator instance
abc <- generate_abc()

A generator instance is an iterator function which yields values:

abc
#> <generator/instance>
#> function() {
#>   for (x in letters[1:3]) {
#>     yield(x)
#>   }
#> }

abc()
#> [1] "a"

Collect all remaining values from an iterator with collect():

collect(abc)
#> [[1]]
#> [1] "b"
#> 
#> [[2]]
#> [1] "c"

Iterate over an iterator with loop():

loop(for (x in generate_abc()) {
  print(toupper(x))
})
#> [1] "A"
#> [1] "B"
#> [1] "C"

See vignette("generator") for more information.

Compatibility with the reticulate package

Python iterators imported with the reticulate package are compatible with loop() and collect():

suppressMessages(library(reticulate))

py_run_string("
def first_n(n):
    num = 1
    while num <= n:
        yield num
        num += 1
")

loop(for (x in py$first_n(3)) {
  print(x * 2)
})
#> [1] 2
#> [1] 4
#> [1] 6

They can also be composed with coro generators:

times <- generator(function(it, n) for (x in it) yield(x * n))

composed <- times(py$first_n(3), 10)

collect(composed)
#> [[1]]
#> [1] 10
#> 
#> [[2]]
#> [1] 20
#> 
#> [[3]]
#> [1] 30

Limitations

yield() and await() can be used in loops, if/else branches, tryCatch() expressions, or any combinations of these. However they can’t be used as function arguments. These will cause errors:

generator(function() {
  list(yield("foo"))
})

async(function() {
  list(await(foo()))
})

Fortunately it is easy to rewrite the code to work around this limitation:

generator(function() {
  x <- yield("foo")
  list(x)
})

async(function() {
  x <- await(foo())
  list(x)
})

How does it work

Coroutines are an abstraction for state machines in languages that support them. Conversely, you can implement coroutines by rewriting the code source provided by the user as a state machine. Pass internals = TRUE to the print methods of coroutines to reveal the state machine that is running under the hood:

print(generate_abc, internals = TRUE)
#> <generator>
#> function() {
#>   for (x in letters[1:3]) {
#>     yield(x)
#>   }
#> }
#> State machine:
#> {
#>     if (exhausted) {
#>         return(invisible(exhausted()))
#>     }
#>     repeat switch(state[[1L]], `1` = {
#>         iterators[[2L]] <- as_iterator(user(letters[1:3]))
#>         state[[1L]] <- 2L
#>         state[[2L]] <- 1L
#>     }, `2` = {
#>         repeat switch(state[[2L]], `1` = {
#>             if ({
#>                 iterator <- iterators[[2L]]
#>                 if (is_exhausted(elt <- iterator())) {
#>                   FALSE
#>                 } else {
#>                   user_env[["x"]] <- elt
#>                   TRUE
#>                 }
#>             }) {
#>                 state[[2L]] <- 2L
#>             } else {
#>                 break
#>             }
#>         }, `2` = {
#>             user({
#>                 x
#>             })
#>             state[[2L]] <- 3L
#>             suspend()
#>             return(last_value())
#>         }, `3` = {
#>             .last_value <- if (missing(arg)) NULL else arg
#>             state[[2L]] <- 1L
#>         })
#>         iterators[[2L]] <- NULL
#>         length(state) <- 1L
#>         break
#>     })
#>     exhausted <- TRUE
#>     invisible(exhausted())
#> }

Despite this transformation of source code, browser() and step-debugging still work as you would expect. This is because coro keeps track of the source references from the original code.

Acknowledgements

The regenerator Javascript package which uses a similar transformation to implement generators and async functions in older versions of Javascript.
Gabor Csardi for many interesting discussions about concurrency and the design of coro.

Installation

Install the development version from github with:

# install.packages("devtools")
devtools::install_github("r-lib/coro", build_vignettes = TRUE)

r-lib/coro

r-lib

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