then: Access the results of a promise
In promises: Abstractions for Promise-Based Asynchronous Programming

View source: R/then.R

then	R Documentation

Access the results of a promise

Description

Use the then function to access the eventual result of a promise (or, if the operation fails, the reason for that failure). Regardless of the state of the promise, the call to then is non-blocking, that is, it returns immediately; so what it does not do is immediately return the result value of the promise. Instead, you pass logic you want to execute to then, in the form of function callbacks (or formulas, see Details). If you provide an onFulfilled callback, it will be called upon the promise's successful resolution, with a single argument value: the result value. If you provide an onRejected callback, it will be called if the operation fails, with a single argument reason: the error that caused the failure.

Usage

then(promise, onFulfilled = NULL, onRejected = NULL)

catch(promise, onRejected, tee = FALSE)

finally(promise, onFinally)

Arguments

`promise`	A promise object. The object can be in any state.
`onFulfilled`	A function (or a formula–see Details) that will be invoked if the promise value successfully resolves. When invoked, the function will be called with a single argument: the resolved value. Optionally, the function can take a second parameter `.visible` if you care whether the promise was resolved with a visible or invisible value. The function can return a value or a promise object, or can throw an error; these will affect the resolution of the promise object that is returned by `then()`.
`onRejected`	A function taking the argument `error` (or a formula–see Details). The function can return a value or a promise object, or can throw an error. If `onRejected` is provided and doesn't throw an error (or return a promise that fails) then this is the async equivalent of catching an error.
`tee`	If `TRUE`, ignore the return value of the callback, and use the original value instead. This is useful for performing operations with side-effects, particularly logging to the console or a file. If the callback itself throws an error, and `tee` is `TRUE`, that error will still be used to fulfill the the returned promise (in other words, `tee` only has an effect if the callback does not throw).
`onFinally`	A function with no arguments, to be called when the async operation either succeeds or fails. Usually used for freeing resources that were used during async operations.

Formulas

For convenience, the then(), catch(), and finally() functions use rlang::as_function() to convert onFulfilled, onRejected, and onFinally arguments to functions. This means that you can use formulas to create very compact anonymous functions, using . to access the value (in the case of onFulfilled) or error (in the case of onRejected).

Chaining promises

The first parameter of then is a promise; given the stated purpose of the function, this should be no surprise. However, what may be surprising is that the return value of then is also a (newly created) promise. This new promise waits for the original promise to be fulfilled or rejected, and for onFulfilled or onRejected to be called. The result of (or error raised by) calling onFulfilled/onRejected will be used to fulfill (reject) the new promise.

promise_a <- get_data_frame_async()
promise_b <- then(promise_a, onFulfilled = head)

In this example, assuming get_data_frame_async returns a promise that eventually resolves to a data frame, promise_b will eventually resolve to the first 10 or fewer rows of that data frame.

Note that the new promise is considered fulfilled or rejected based on whether onFulfilled/onRejected returns a value or throws an error, not on whether the original promise was fulfilled or rejected. In other words, it's possible to turn failure to success and success to failure. Consider this example, where we expect some_async_operation to fail, and want to consider it an error if it doesn't:

promise_c <- some_async_operation()
promise_d <- then(promise_c,
  onFulfilled = function(value) {
    stop("That's strange, the operation didn't fail!")
  },
  onRejected = function(reason) {
    # Great, the operation failed as expected
    NULL
  }
)

Now, promise_d will be rejected if promise_c is fulfilled, and vice versa.

Warning: Be very careful not to accidentally turn failure into success, if your error handling code is not the last item in a chain!

some_async_operation() %>%
  catch(function(reason) {
    warning("An error occurred: ", reason)
  }) %>%
  then(function() {
    message("I guess we succeeded...?")  # No!
  })

In this example, the catch callback does not itself throw an error, so the subsequent then call will consider its promise fulfilled!

Convenience functions

For readability and convenience, we provide catch and finally functions.

The catch function is equivalent to then, but without the onFulfilled argument. It is typically used at the end of a promise chain to perform error handling/logging.

The finally function is similar to then, but takes a single no-argument function (or formula) that will be executed upon completion of the promise, regardless of whether the result is success or failure. It is typically used at the end of a promise chain to perform cleanup tasks, like closing file handles or database connections. Unlike then and catch, the return value of finally is ignored; however, if an error is thrown in finally, that error will be propagated forward into the returned promise.