[This article was first published on Colin Fay, and kindly contributed to R-bloggers]. (You can report issue about the content on this page here)
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

Create your own `safely`, `compose` and friends!

## What is an adverb

If you read carefully the purrr documentation, you’ll find this simple explanation :

Adverbs modify the action of a function; taking a function as input and returning a function with modified action as output.

In other words, adverbs take a function, and return this function modified. Yes, just as an adverb modifies a verb. So if you do :

```library(purrr)
safe_log <- safely(log)
```

The returned object is another function that you can use just as a regular one.

```class(safe_log)

## [1] "function"

safe_log("a")

## \$result
## NULL
##
## \$error
## <simpleError in log(x = x, base = base): argument non numérique pour une fonction mathématique>
```

In computer science, these adverbs are what is called “high-order functions”.

## How to write your own?

I’ve been playing with adverbs in {attempt}, notably through these adverbs :

```library(attempt)

# Silently only return the errors, and nothing if the function succeeds
silent_log <- silently(log)
silent_log(1)

# Surely make a function always work, without stopping the process
sure_log <- surely(log)
sure_log(1)

## [1] 0

sure_log("a")

# with_message and with_warning
as_num_msg <- with_message(as.numeric, msg = "We're performing a numeric conversion")
as_num_warn <- with_warning(as.numeric, msg = "We're performing a numeric conversion")
as_num_msg("1")

## We're performing a numeric conversion

## [1] 1

as_num_warn("1")

## Warning in as_num_warn("1"): We're performing a numeric conversion

## [1] 1
```

So, how to implement this kind of behavior? Let’s take a simple example with `sleepy`, also shared on Twitter.

```sleepy <- function(fun, sleep){
function(...){
Sys.sleep(sleep)
fun(...)
}
}

sleep_print <- sleepy(Sys.time, 5)
class(sleep_print)

## [1] "function"

# Let's try
Sys.time()

## [1] "2018-04-19 10:20:58 CEST"

sleep_print()

## [1] "2018-04-19 10:21:03 CEST"
```

Let’s decompose what we’ve got here.

First of all, the function should return another function, so we need to start with :

```talky <- function(){
function(){

}
}
```

What this function will take as a first argument is another function, that will be executed when our future new function is called.

So let’s do this:

```talky <- function(fun){
function(){
fun()
}
}
```

Because you know, with R referential transparency, you can create a variable that is a function:

```plop <- mean
plop(1:10)

## [1] 5.5
```

This simple skeleton will work if we take a function without any args:

```sys_time <- talky(Sys.time)
sys_time()

## [1] "2018-04-19 10:21:03 CEST"
```

But hey, this is not what we want: we need this new function to be able to take arguments. So let’s use our friend `...`.

```talky <- function(fun){
function(...){
fun(...)
}
}
```

Now, our new adverb creates a function that can take arguments. But as you’ve notice, this is still not really an adverb: we need to modify something. Now you’re only limited by your imagination 😉

```# Print the time
talky <- function(fun){
function(...){
print(Sys.time())
fun(...)
}
}
talky_sqrt <- talky(sqrt)
talky_sqrt(10)

## [1] "2018-04-19 10:21:03 CEST"

## [1] 3.162278

# Or with a kind message ?
talky <- function(fun, mess){
function(...){
message(mess)
fun(...)
}
}
talky_sqrt<- talky(fun = sqrt, mess = "Hey there! You Rock!")
talky_sqrt(1)

## Hey there! You Rock!

## [1] 1

# Run it or not ?
maybe <- function(fun){
function(...){
num <- sample(1:100, 1)
if (num > 50) {
fun(...)
}
}
}
maybe_sqrt <- maybe(fun = sqrt)
maybe_sqrt(1)
maybe_sqrt(1)

## [1] 1

maybe_sqrt(1)

## [1] 1

# Create a log file of a function
log_calls <- function(fun, file){
function(...){
write(as.character(Sys.time()), file, append = TRUE, sep = "\n")
fun(...)
}
}
log_sqrt <- log_calls(sqrt, file = "logs")
log_sqrt(10)

## [1] 3.162278

log_sqrt(13)

## [1] 3.605551