Perform an while-like loop in a pipeline

Allows the user to perform a while-like loop without breaking out of a pipeline. To maintain the flow of a pipeline, it is recommended to use [fseq][] style arguments (i.e. pipelines) for the fun argument.

Within the while_pipe(), users can use the .counter variable to reference how many iterations have been performed so far.

while_pipe(data, cond, fun)

Arguments

data	the data being passed through the pipeline.
cond	an expression to be evaluated in the context of data to decide whether to perform fun. Should evaluate to a single logical value or be coercible to one.
fun	pipeline to perform until cond is FALSE.

Examples

sample(100, 1) %>%
  runif() %>%
  pipe_cat("Current length: ", length(.), "\n") %>%
  while_pipe(
    length(.) > 1,
    . %>%
      diff() %>%
      magrittr::divide_by(2)
  )
#> Current length:  95 
#> [1] 0.009624105


tibble::tibble(x = runif(5)) %>%
  while_pipe(
    .counter <= 5,
    . %>%
      dplyr::mutate(!!paste0("x_", .counter) := x - x[.counter])
  )
#> # A tibble: 5 x 6
#>        x      x_1   x_2      x_3     x_4    x_5
#>    <dbl>    <dbl> <dbl>    <dbl>   <dbl>  <dbl>
#> 1 0.803   0       0.780  0.00803 -0.0831  0.203
#> 2 0.0239 -0.780   0     -0.772   -0.863  -0.576
#> 3 0.795  -0.00803 0.772  0       -0.0912  0.195
#> 4 0.887   0.0831  0.863  0.0912   0       0.287
#> 5 0.600  -0.203   0.576 -0.195   -0.287   0