<%@meta language="R-vignette" content="-------------------------------- %\VignetteIndexEntry{A Future for R: Text and Message Output} %\VignetteAuthor{Henrik Bengtsson} %\VignetteKeyword{R} %\VignetteKeyword{package} %\VignetteKeyword{vignette} %\VignetteKeyword{future} %\VignetteKeyword{promise} %\VignetteKeyword{output} %\VignetteKeyword{standard output} %\VignetteKeyword{stdout} %\VignetteKeyword{standard error} %\VignetteKeyword{stderr} %\VignetteKeyword{message} %\VignetteKeyword{condition} %\VignetteEngine{R.rsp::rsp} %\VignetteTangle{FALSE} --------------------------------------------------------------------"%> # <%@meta name="title"%> Futures will _relay_ output produced by functions such as `cat()`, `print()` and `str()`. More specifically, output sent to the standard output (stdout) while a future is evaluated will be captured and _re-outputted ("relayed") when the value of the future is queried_. Messages produced by `message()`, which formally are R conditions are also captured and resignaled ("relayed") as messages in the main R session. Importantly, this works identically regardless of future backend used. For simplicitly, lets start with an illustration on how standard output ("stdout") is captured and relayed: ```r > library(future) > plan(multisession) > fa <- future({ cat("Hello world!\n"); print(1:3); 42L }) > fb <- future({ str(iris); summary(iris) }) > a <- value(fa) Hello world! [1] 1 2 3 > b <- value(fb) 'data.frame': 150 obs. of 5 variables: $ Sepal.Length: num 5.1 4.9 4.7 4.6 5 5.4 4.6 5 4.4 4.9 ... $ Sepal.Width : num 3.5 3 3.2 3.1 3.6 3.9 3.4 3.4 2.9 3.1 ... $ Petal.Length: num 1.4 1.4 1.3 1.5 1.4 1.7 1.4 1.5 1.4 1.5 ... $ Petal.Width : num 0.2 0.2 0.2 0.2 0.2 0.4 0.3 0.2 0.2 0.1 ... $ Species : Factor w/ 3 levels "setosa","versicolor",..: 1 1 1 1 1 1 1 1 1 1 ... > a [1] 42 > b Sepal.Length Sepal.Width Petal.Length Petal.Width Species Min. :4.300 Min. :2.000 Min. :1.000 Min. :0.100 setosa :50 1st Qu.:5.100 1st Qu.:2.800 1st Qu.:1.600 1st Qu.:0.300 versicolor:50 Median :5.800 Median :3.000 Median :4.350 Median :1.300 virginica :50 Mean :5.843 Mean :3.057 Mean :3.758 Mean :1.199 3rd Qu.:6.400 3rd Qu.:3.300 3rd Qu.:5.100 3rd Qu.:1.800 Max. :7.900 Max. :4.400 Max. :6.900 Max. :2.500 ``` Note that the captured standard output (stdout) will be relayed _each_ time `value()` is called, e.g. ```r > a <- value(fa) Hello world! [1] 1 2 3 > a <- value(fa) Hello world! [1] 1 2 3 ``` Output is relayed the same way when using future assignments (`%<-%`). For example, ```r > library(future) > plan(multisession) > a %<-% { cat("Hello world!\n"); print(1:3); 42L } > b %<-% { str(iris); summary(iris) } > a Hello world! [1] 1 2 3 [1] 42 > b 'data.frame': 150 obs. of 5 variables: $ Sepal.Length: num 5.1 4.9 4.7 4.6 5 5.4 4.6 5 4.4 4.9 ... $ Sepal.Width : num 3.5 3 3.2 3.1 3.6 3.9 3.4 3.4 2.9 3.1 ... $ Petal.Length: num 1.4 1.4 1.3 1.5 1.4 1.7 1.4 1.5 1.4 1.5 ... $ Petal.Width : num 0.2 0.2 0.2 0.2 0.2 0.4 0.3 0.2 0.2 0.1 ... $ Species : Factor w/ 3 levels "setosa","versicolor",..: 1 1 1 1 1 1 1 1 1 1 ... Sepal.Length Sepal.Width Petal.Length Petal.Width Species Min. :4.300 Min. :2.000 Min. :1.000 Min. :0.100 setosa :50 1st Qu.:5.100 1st Qu.:2.800 1st Qu.:1.600 1st Qu.:0.300 versicolor:50 Median :5.800 Median :3.000 Median :4.350 Median :1.300 virginica :50 Mean :5.843 Mean :3.057 Mean :3.758 Mean :1.199 3rd Qu.:6.400 3rd Qu.:3.300 3rd Qu.:5.100 3rd Qu.:1.800 Max. :7.900 Max. :4.400 Max. :6.900 Max. :2.500 ``` Note how the captured output is relayed followed by the printing of the value. Also, since the future value is only queried once when using future assignments, or more specifically when using promises, the output is only relayed once. For example, querying `a` again will only print its value, because it is now a regular R object: ```r > a [1] 42 > a [1] 42 ``` Next, lets see what happens if we use `message()` to produce output: ```r > library(future) > plan(multisession) > fa <- future({ message("Hello world!"); 42L }) > value(fa) Hello world! [1] 42 ``` Note that contrary to the captured stdout, which is captured as one single block output, messages are conditions that are captured separately. Unfortunately, it is _not_ possible to preserve the ordering of interweaved stdout and message output. When using futures, stdout output will always be relayed first followed by each of the individual conditions captured. For example, ```r > library(future) > plan(multisession) > fa <- future({ message("Hello"); print(1:3); message("world!"); cat("ping\n"); 42L }) > value(fa) [1] 1 2 3 ## <= stdout as a single ... ping ## <= ... block of output Hello ## <= 1st message world! ## <= 2nd message [1] 42 ``` ## Future frontends The output is relayed automatically also when using frontends such as [future.apply] or [foreach] with [doFuture]. Again, it works with any future backend. For example, ```r > library(future.apply) > plan(future.callr::callr) > y <- future_lapply(1:3, FUN = function(x) { cat("x =", x, "\n"); message("x : ", x); sqrt(x) }) x = 1 x = 2 x = 3 x : 1 ## <= 1st message x : 2 ## <= 2nd message x : 3 ## <= 3rd message > str(y) List of 3 $ : num 1 $ : num 1.41 $ : num 1.73 ``` Equivalently, ```r > library(doFuture) > registerDoFuture() > plan(future.callr::callr) > y <- foreach(x = 1:3) %dopar% { cat("x =", x, "\n"); message("x : ", x); sqrt(x) } x = 1 x = 2 x = 3 x : 1 ## <= 1st message x : 2 ## <= 2nd message x : 3 ## <= 3rd message > str(y) List of 3 $ : num 1 $ : num 1.41 $ : num 1.73 ``` ## Capturing output To capture the output produced by futures, use `capture.output()` as you would do when capturing output elsewhere in R. For example, ```r > library(future) > fa <- future({ cat("Hello world!\n"); print(1:3); 42L }) > stdout <- capture.output(a <- value(fa)) > stdout [1] "Hello world!" "[1] 1 2 3" > a [1] 42 ``` ## Suppressing messages ```r > library(future) > plan(multisession) > fa <- future({ message("Hello"); print(1:3); message("world!"); cat("ping\n"); 42L }) > suppressMessages(a <- value(fa)) [1] 1 2 3 ping > a [1] 42 ``` ## Known limitations It is only the standard output that is relayed. It is _not possible_ to relay output send to the standard error (stderr), e.g. output by `cat(..., file = stderr())` will be lost. This is due to a [limitation in R](https://github.com/HenrikBengtsson/Wishlist-for-R/issues/55), preventing us from capturing stderr in a reliable way, particularity across all backends. However, note that the captured messages by `message()` are outputted to stderr (as expected) when resignaled/relayed. [foreach]: https://cran.r-project.org/package=foreach [future]: https://cran.r-project.org/package=future [future.apply]: https://cran.r-project.org/package=future.apply [doFuture]: https://cran.r-project.org/package=doFuture [globals]: https://cran.r-project.org/package=globals [listenv]: https://cran.r-project.org/package=listenv