capturing expressions and evaluating later

capture.R

library(tidyverse)

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Base R styel
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
filter_base <- function(df, boolean_expression) {

  # Don't evaluate what we were given. Just hold it.
  captured_expression <- substitute(boolean_expression)

  # with df as the environment, now evaluate the expression
  keep_rows <- with(df, eval(captured_expression))

  # use those values for whatever
  df[keep_rows, ]
}

filter_base(mtcars, cyl==8)




#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Tidyverse/rlang/quasiquotation style
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
filter_tidy <- function(df, boolean_expression) {

  # Don't evaluate what we were given. Just hold it.
  captured_expression <- enquo(boolean_expression)

  # evaluate it within a function that is aware of the quasiquotation stuff
  filter(df, !!captured_expression)
}


filter_tidy(mtcars, cyl==8)