If a frame or formula, the function will be retrieved from the associated environment. Otherwise, it is looked up in the calling frame.
lang_fn(lang)
Can be a call (language object), a formula quoting a call in the right-hand side, or a frame object from which to extract the call expression.
# NOT RUN {
# Extract from a quoted call:
lang_fn(~matrix())
lang_fn(quote(matrix()))
# Extract the calling function
test <- function() lang_fn(call_frame())
test()
# }
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