How to use context

library(context)
#> 
#> Attaching package: 'context'
#> The following objects are masked from 'package:base':
#> 
#>     open, with

Basic usage

Usage will be familiar to anyone that used Pythons with-as context manager. For example, to iterate file line by line, run a function on a line, then do something with the result:

# Add numbers 1, 2, 3, 4, 5 to a file
set.seed(42)
temp_file <- tempfile()
writeLines(as.character(1:5), temp_file)
numbers <- c()

with(open(temp_file) %as% f, {
  while (read_line(f) %as% line) {
    numbers <- c(numbers, as.integer(line))
  }
})

cat("Sum of the numbers is: ", sum(numbers), "\n")
#> Sum of the numbers is:  15
file.remove(temp_file) |> invisible()

In addition to the with context creation, %as% can be used to store the value of the expression on its left side to the variable name on its right side. When used in this manner, this type of assignment returns FALSE only when the end of the is reached, that is when the length of the return value is zero. This is useful when iterating through files in chunks; for example line by line or block-by-block as is the case for example with ShortRead::FastqStreamer().