In parsermd: Formal Parser and Related Tools for R Markdown Documents

Three arguments in chunk hooks

A chunk hook has three arguments: before, options and envir. We show how they work through some simple examples.

The `before` argument

It is a logical argument: before == TRUE executes code before a chunk.

library(knitr)
knit_hooks$set(foo1 = function(before, options, envir) {
  if (before) {
    '_I appear before a chunk!_\n\n'
  } else {
    '\n\n_I am after a chunk..._'
  }
})

Test the foo1 hook:

1+1

The `options` argument

It contains all the chunk options (include global options) for the current chunk.

knit_hooks$set(foo2 = function(before, options, envir) {
  if (!before) {
    z = capture.output(str(options[c('eval', 'dev', 'results', 'bar1', 'bar2', 'bar3')]))
    z = paste('    ', z, sep = '', collapse = '\n')
    paste('Some chunk options in the above chunk are:\n\n', z, sep = '')
  }
})

Test the foo2 hook:

1+1

The `envir` argument

It is the environment of the current chunk.

knit_hooks$set(foo3 = function(before, options, envir) {
  if (!before) {
    paste('Objects available in the above chunk:',
          paste('`', ls(envir), '`', sep = '', collapse = ', '))
  }
})

Test the foo3 hook:

x2=1+1; y3=rnorm(10)

Another example:

knit_hooks$set(foo4 = function(before, options, envir) {
  if (!before && exists('z5', envir = envir)) {
    sprintf('**Ha! I see z5 = %.3f!**', envir$z5)
  }
})