s7-promises.md

library(S7)

# problem -----------------------------------------------------------------

class_a <- new_class("class_a", properties = list(x = class_character))
class_b <- new_class(
  "class_b",
  properties = list(class_a = class_a),
  constructor = function(class_a = class_a()) {
    new_object(S7_object(), class_a = class_a)
  }
)
try(class_b())
#> Error in new_object(S7_object(), class_a = class_a) : 
#>   promise already under evaluation: recursive default argument reference or earlier problems?


# alias -------------------------------------------------------------------

class_a <- new_class("class_a", properties = list(x = class_character))
.class_a <- function() class_a()
class_b <- new_class(
  "class_b",
  properties = list(class_a = class_a),
  constructor = function(class_a = .class_a()) {
    new_object(S7_object(), class_a = class_a)
  }
)
class_b()
#> <class_b>
#>  @ class_a: <class_a>
#>  .. @ x: chr(0)


# get() -------------------------------------------------------------------

class_a <- new_class("class_a", properties = list(x = class_character))
class_b <- new_class(
  "class_b",
  properties = list(class_a = class_a),
  constructor = function(class_a = get("class_a", 1)()) {
    new_object(S7_object(), class_a = class_a)
  }
)
class_b()
#> <class_b>
#>  @ class_a: <class_a>
#>  .. @ x: chr(0)

^{Created on 2023-10-09 with reprex v2.0.2.9000}