rand_seed: An unevaluated expression to return .Random.seed if exists
Description
This expression returns .Random.seed when eval(rand_seed) and
NULL otherwise.
Usage
rand_seed
Arguments
format
language { if (exists(".Random.seed", envir = globalenv())) ; get(".Random.seed", envir = globalenv()) }
Details
It is designed to work with opts_chunk$set(cache.extra = rand_seed)
for reproducibility of chunks that involve with random number generation. See
references.