simmer: reading resources from inside trajectory functions - r

Question

I want to be able to modify the resource capacity inside trajectory as a function of queue length.
The following (simplified) code below does not work. - When I try to call get_mon_resources(simStore) inside the function, the code crashes with the error:
Error in run_(private$sim_obj, until) :
Expecting a single value: [extent=0].
Thank you for your help.
simStore <- simmer()
fUpdateNumberOfCashiers <- function() {
dtLastRes <- simStore %>% get_mon_resources %>% tail(1)
nCapacityNow <- dtLastRes$capacity # same result with get_capacity(simStore),
nQueueNow <- dtLastRes$queue # same result with get_queue_count(simStore)
print(dtLastRes) # prints empty data-frame !
return (5) # crashes here ! (eventually 5 will be replaced with more meaningful formula
}
trajClient <- trajectory("Client's path") %>%
log_("Arrived to cashier") %>%
set_capacity("Cashier", value = fUpdateNumberOfCashiers ) %>%
seize("Cashier") %>%
timeout(function() {rexp(1, 30)}) %>% # One Cashier processes 30 clients / hour
release("Cashier") %>%
log_(function(attr) { sprintf("In total spent %.2f", now(simStore) - attr["start_time"])})
simStore <- simmer("Store") %>%
add_resource("Cashier", 1) %>%
add_generator("Store Clients", trajClient, function() {rexp(1, 120)}) %>% # 120 clients / hour
run(until=nHoursObserved <- 1) ; simStore

Answer 1

See the discussion related to troubleshooting this problem here: https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!topic/simmer-devel/NgIikOpHpss
What causes the problem is that the other package (lubridate) masks objects from "simmer", as written seen below:
Attaching package: ‘lubridate’
The following objects are masked from ‘package:simmer’:
now, rollback
Once I replaced
library(simmer); library(lubridate);
with
library(lubridate); library(simmer);
The problem disappeared!