R furrr plan returns errors when trying to tweak the multisession plan?

Question

I am trying to set the plan I need but I am getting the following errors:

no_cores <- availableCores() - 2
plan(multisession, workers = no_cores, lazy = T, gc = T)

and the error is:

Error in MultisessionFuture(expr = expr, envir = envir, substitute = FALSE,  : 
  argument "expr" is missing, with no default

or:

plan(multisession, workers = no_cores, lazy = T, gc = T)
Error in tweak.future(function (expr, envir = parent.frame(), substitute = TRUE,  : 
  Future argument 'lazy' must not be tweaked / set via plan()

Please advise how can I set the workers, lazy, gc and other parameters of multisession/multicore plans.

My R version is:

R.Version()
$platform
[1] "x86_64-pc-linux-gnu"

$arch
[1] "x86_64"

$os
[1] "linux-gnu"

$system
[1] "x86_64, linux-gnu"

$status
[1] ""

$major
[1] "4"

$minor
[1] "0.2"

$year
[1] "2020"

$month
[1] "06"

$day
[1] "22"

$`svn rev`
[1] "78730"

$language
[1] "R"

$version.string
[1] "R version 4.0.2 (2020-06-22)"

$nickname
[1] "Taking Off Again"

Answer 1

Trying your example I've got the second error message. And I've basically followed the advice given: you set the argument lazy = T in plan which is not allowed. However, you can set this argument directly in the furrr function call:

library(furrr)
no_cores <- availableCores() - 2
plan(multisession, workers = no_cores, gc = T)

future_map(c("hello", "world"), ~.x,
           .options = future_options(lazy = TRUE))
[[1]]
[1] "hello"

[[2]]
[1] "world"