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%\VignetteIndexEntry{A Future for R: Text and Message Output}
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# <%@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(multiprocess)

> 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(multiprocess)

> 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 relaid 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(multiprocess)
> 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(multiprocess)
> 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
```


## Supressing messages

```r
> library("future")
> plan(multiprocess)
> 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 resignalled/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

---
Copyright Henrik Bengtsson, 2018-2019
