magrader

magrader creates a simple checking scheme based on magrittr pipes for tutor tutorials.

With magrader, authors write checks as pipes that pass student submissions from one check to the next. Authors can refer to the submission as .answer (which could be complemented in the future with .result), and they can do this in a natural way: magrader hides the non-standard evaluation that occurs behind the scenes. In other words, a tutor author can directly manipulate the student answer as if they were using magrittr pipes in a normal R environment.

magrader is an experiment, a proof-of-concept. As a result, it only implements one checking function.

Examples

magrader supplies the code_calls() function, which checks whether a piece of student code calls an R function or object. To use code_calls(), recreate the exercise and -check chunk in the demo tutor file.

To check student code with code_calls():

1. Add to the tutorial's setup chunk:

   library(tutor)
   library(magrader)
   library(magrittr)
   tutor_options(exercise.checker = pipe_checker)

2. Write a pipe that begins with .answer in an exercise's -check chunk, e.g.

   .answer %>% 
     code_calls("mtcars")

magrader will run each check in the pipe, in order, on the code. Let's assume that the student's answer in the tutor exercise consisted of exactly mtcars. Then the short pipe above will return the result:

To add multiple checks, extend the pipe. Notice that magrader will skip downstream checks as soon as a check returns a failure. So the checking code below would return the image that follows

.answer %>% 
  code_calls("mpg") %>% 
  code_calls("mtcars") 

You can customise the failure message with message:

.answer %>% 
  code_calls("mpg", message = "You should use mpg instead.") %>% 
  code_calls("mtcars") 

Check failures need not be catastrophic. Set warn = TRUE to treat the failure as a warning. In this case, magrader will run downstream checks. As long as the code does not fail a downstream check, the warning message will be printed as part of its feedback.

.answer %>% 
  code_calls("mpg", message = "You should use mpg instead.", warn = TRUE) %>% 
  code_calls("mtcars") 

You can also praise your students for right answers, or provide inocuous comments with praise. When appropriate, magrader will combine multiple messages.

.answer %>% 
  code_calls("mtcars", praise = "I'm glad you didn't use mpg.") %>% 
  code_calls("mtcars", praise = "On second thought, I'm really glad.")