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stack_trim
Trim top call layers from the evaluation stack
Description
ctxt_stack() can be tricky to use in real code because all
intervening frames are returned with the stack, including those at
ctxt_stack() own call site. stack_trim() makes it easy to
remove layers of intervening calls.
Usage
stack_trim(stack, n = 1)
Arguments
stack |
An evaluation stack. |
n |
The number of call frames (not eval frames) to trim off the top of the stack. In other words, the number of layers of intervening frames to trim. |
Life cycle
These functions are deprecated and replaced by trace_back().
Examples
# Intervening frames appear on the evaluation stack:
identity(identity(ctxt_stack()))
# stack_trim() will trim the first n layers of calls:
stack_trim(identity(identity(ctxt_stack())))
# Note that it also takes care of calls intervening at its own call
# site:
identity(identity(
stack_trim(identity(identity(ctxt_stack())))
))
# It is especially useful when used within a function that needs to
# inspect the evaluation stack but should nonetheless be callable
# within nested calls without side effects:
stack_util <- function() {
# n = 2 means that two layers of intervening calls should be
# removed: The layer at ctxt_stack()'s call site (including the
# stack_trim() call), and the layer at stack_util()'s call.
stack <- stack_trim(ctxt_stack(), n = 2)
stack
}
user_fn <- function() {
# A user calls your stack utility with intervening frames:
identity(identity(stack_util()))
}
# These intervening frames won't appear in the evaluation stack
identity(user_fn())