Difference in behaviour for dynamic branching [help]

[grimmjulian]

Help

• I understand and agree to https://books.ropensci.org/targets/help.html.

Description

If I start dynamic branching over a list,
I need to subset the first target one more time.
If the dynamic branching is started over a vector,
this is not necessary.

I created the following example.
The target a* should be equivalent to b* IMO,
target o is just a placeholder to start with a vector.

My question is:
Is this the intended behavior or is my usage not correct?

library(targets)

list(
	tar_target(
		name = a,
		command = {
			lapply(
				1:3,
				\(x) list(x = letters[x], y = LETTERS[x])
			)
		}
		# this target is the list with three sublist
		# each sublist containing each two elements
		# everything is as expected
	),
	tar_target(
		name = aa,
		command = a[[1]],
		pattern = map(a),
		iteration = "list"
		# here the list returns the same except for the names
		# testthat::expect_equal(
		#   targets::tar_read(a), unname(targets::tar_read(aa))
		#  ) is true
	),
	tar_target(
		name = ab,
		command = a[[1]],
		pattern = map(a)
		# here the list is flattened for some reason
	),
	tar_target(
		name = aaa,
		command = aa[[1]],
		pattern = map(aa),
		iteration = "list"
		# so why do i need to to do [[1]][[1]]
		# to get the first element of each sublist?
	),
	tar_target(
		name = o,
		command = 1:3
	),
	tar_target(
		name = b,
		command = list(x = letters[o], y = LETTERS[o]),
		pattern = map(o),
		iteration = "list"
		# so this should be equivalent to target a right?
		# testthat::expect_equal(
		#   targets::tar_read(a), unname(targets::tar_read(b))
	  # ) is true
	),
	tar_target(
		name = ba,
		command = b[[1]],
		pattern = map(b),
		iteration = "list"
		# This should be equivalent to target aa without the names?
		# so why does this return the first element of each element and not each sublist?
		# This is equivalent to aaa.
	)
)

[wlandau]

The general behavior you describe is intended because the default iteration argument of tar_target() is "vector", so slicing happens with vctrs::vec_slice(). vec_slice() behaves like [. If you want list indexing with [[, choose iteration = "list". Details: https://books.ropensci.org/targets/dynamic.html#iteration

[grimmjulian]

Thanks, I somehow did not realize that the argument can be used for splicing and aggregation!