• on_load() registers expressions to be run on the user's machine each time the package is loaded in memory. This is by contrast to normal R package code which is run once at build time on the packager's machine (e.g. CRAN).

    on_load() expressions require run_on_load() to be called inside .onLoad().

  • on_package_load() registers expressions to be run each time another package is loaded.

on_load() is for your own package and runs expressions when the namespace is not sealed yet. This means you can modify existing binding or create new ones. This is not the case with on_package_load() which runs expressions after a foreign package has finished loading, at which point its namespace is sealed.

on_load(expr, env = parent.frame(), ns = topenv(env))

run_on_load(ns = topenv(parent.frame()))

on_package_load(pkg, expr, env = parent.frame())



An expression to run on load.


The environment in which to evaluate expr. Defaults to the current environment, which is your package namespace if you run on_load() at top level.


The namespace in which to hook expr.


Package to hook expression into.

When should I run expressions on load?

There are two main use cases for running expressions on load:

  1. When a side effect, such as registering a method with s3_register(), must occur in the user session rather than the package builder session.

  2. To avoid hard-coding objects from other packages in your namespace. If you assign foo::bar or the result of foo::baz() in your package, they become constants. Any upstream changes in the foo package will not be reflected in the objects you've assigned in your namespace. This often breaks assumptions made by the authors of foo and causes all sorts of issues.

    Recreating the foreign objects each time your package is loaded makes sure that any such changes will be taken into account. In technical terms, running an expression on load introduces indirection.

Comparison with .onLoad()

on_load() has the advantage that hooked expressions can appear in any file, in context. This is unlike .onLoad() which gathers disparate expressions in a single block.

on_load() is implemented via .onLoad() and requires run_on_load() to be called from that hook.


quote({ # Not run # First add `run_on_load()` to your `.onLoad()` hook, # then use `on_load()` anywhere in your package .onLoad <- function(lib, pkg) { run_on_load() } # Register a method on load on_load(s3_register("foo::bar", "my_class")) # Assign an object on load var <- NULL on_load( var <- foo() ) # To use `on_package_load()` at top level, wrap it in `on_load()` on_load(on_package_load("foo", message("foo is loaded"))) # In functions it can be called directly f <- function() on_package_load("foo", message("foo is loaded")) })
#> { #> .onLoad <- function(lib, pkg) { #> run_on_load() #> } #> on_load(s3_register("foo::bar", "my_class")) #> var <- NULL #> on_load(var <- foo()) #> on_load(on_package_load("foo", message("foo is loaded"))) #> f <- function() on_package_load("foo", message("foo is loaded")) #> }