R treasures: modifyList()

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Sometimes, more precisely quite often, the standard libraries hold treasures that we are not aware of.
Maybe they have obscure names, have been reinvented and shadowed by the newest cutting edge convenience package or one simply does not expect those treasures to be there so they are hidden in plain sight.

The modifyList() function is one of those treasures. Its part of the {utils} packages so it ships with every R version ready to use without further dependencies.

The function offers a way to merge two lists into one similar to c(), the concatenate function.
But other than with using c() items with the same keys will be updated instead of simply added.
Thus modifyList() presents the answer to the question:

In R, how to update items of a list with the values of another list?

Let’s first create two lists that will serve as example.

list_a <- list(key_1 = 1:3)
## $key_1
## [1] 1 2 3

list_b <- list(key_1 = 7:9, key_2 = "my_string")
## $key_1
## [1] 7 8 9
## $key_2
## [1] "my_string"

Now we can first look at what happens when using c() to combine the two lists.
True to its name, the concatenate function, combines the two lists such that every
item is part of the newly created list.

c(list_a, list_b)
## $key_1
## [1] 1 2 3
## $key_1
## [1] 7 8 9
## $key_2
## [1] "my_string"

Now let’s use modifyList() on our example data. Instead of three items we now only get two.
while the second item of list_b has simply been added to the new list, the first item
of list_a has been updated (modified) with the value of the item in list_b.

modifyList(list_a, list_b)
## $key_1
## [1] 7 8 9
## $key_2
## [1] "my_string"


There are some caveats to note however. First, let us have a look at the function

A modified version of x, with the modifications determined as follows (here, list elements are identified by their names). Elements in val which are missing from x are added to x. For elements that are common to both but are not both lists themselves, the component in x is replaced (or possibly deleted, depending on the value of keep.null) by the one in val. For common elements that are in both lists, x[[name]] is replaced by modifyList(x[[name]], val[[name]])

The documentation basically describes the behavior we have already observed in our
example but it also mentions the keep.null parameter that per default is set
to FALSE. This parameter emulates the behavior we are used to when deleting items
from a list by assigning NULL to that item, e.g.: list['key_1'] <- NULL.
If we want to update a list with another list that has embedded NULL values
that we want to keep, the deletion of those items might come unexpected
(on the other hand the value of a none existent item will always be NULL anyways, e.g.:
list()$a, list()[['a']]).

The second caveat is that modifyList() will not update unnamed items of a list.
This makes sense if you think of modifyList() as unnamed having no name to match
values upon. Again, its mostly just a good idea to know, that’s that the way the
function works.

Use Cases

Now what is this useful for? I myself often use modifyList() when I want to
pass around whole sets of information which

(1) which have a default set of values and
(2) might get extended in the future.

That way I can always add options while older versions of options will still
work by extending the default options and later on updating the default values
with those values I specifiacally want to differ from the defaults.

options_default <- 
        plots = TRUE,
        font_face = "Comic Sans",
        author = "No One In Particular"

options_patch <- 
        author = "Me MySelf And I"
options_to_use <- modifyList(options_default, options_patch)
## $plots
## [1] TRUE
## $font_face
## [1] "Comic Sans"
## $author
## [1] "Me MySelf And I"

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