Stepping through a pipeline with intermediate results

Add print:

mtcars %>% 
  group_by(cyl) %>% 
  print %>% 
  sample_frac(0.1) %>% 
  print %>% 
  summarise(res = mean(mpg))

You can select which results to print by using the tee-operator (%T>%) and print(). The tee-operator is used exclusively for side-effects like printing.

# i.e.
mtcars %>%
  group_by(cyl) %T>% print() %>%
  sample_frac(0.1) %T>% print() %>%
  summarise(res = mean(mpg))

It is easy with magrittr function chain. For example define a function my_chain with:

foo <- function(x) x + 1
bar <- function(x) x + 1
baz <- function(x) x + 1
my_chain <- . %>% foo %>% bar %>% baz

and get the final result of a chain as:

     > my_chain(0)
    [1] 3

You can get a function list with functions(my_chain) and define a "stepper" function like this:

stepper <- function(fun_chain, x, FUN = print) {
  f_list <- functions(fun_chain)
  for(i in seq_along(f_list)) {
    x <- f_list[[i]](x)
    FUN(x)
  }
  invisible(x)
}

And run the chain with interposed print function:

stepper(my_chain, 0, print)

# [1] 1
# [1] 2
# [1] 3

Or with waiting for user input:

stepper(my_chain, 0, function(x) {print(x); readline()})