An error checking function for use with learnr

learnr uses the checking code in exercise.error.check.code when the user's submission produces an error during evaluation. gradethis_error_checker() provides default error checking suitable for most situations where an error was not expected.

If a solution for the exercise is available, the user's submission will be compared to the example solution and the message to the student will include code feedback. Otherwise, the error message from R is returned.

If you are expecting the user to submit code that throws an error, use the *-error-check chunk to write custom grading code that validates that the correct error was created.

Usage

gradethis_error_checker(
  ...,
  message = getOption("gradethis.error_checker.message", NULL),
  encourage = getOption("gradethis.fail.encourage", FALSE)
)

Arguments

...

Ignored but included for future compatibility.

message

The feedback message when an error occurred and no solution is provided for the exercise. May reference .error or any of the grade_this-objects. The default value is set by gradethis_setup().

encourage

Include a random encouraging phrase with random_encouragement()? The default value of encourage can be set using gradethis_setup() or the gradethis.fail.encourage option.

Value

A checking function compatible with gradethis_exercise_checker().

See also

gradethis_setup(), gradethis_exercise_checker()

Examples

# The default error checker is run on an exercise that produces an error.
# In the following example, the object `b` is not defined.

# This is the error that the user's submission creates:
tryCatch(
  b,
  error = function(e) message(e$message)
)
#> object 'b' not found

# If you haven't provided a model solution:
gradethis_error_checker()(mock_this_exercise(b))
#> <gradethis_graded: [Incorrect]
#>   An error occurred with your R code:
#> 
#>   ```
#>   object 'b' not found
#>   ```
#> 
#>   Try it again. You get better each time.
#> >

# If a model solution is available:
gradethis_error_checker()(mock_this_exercise(b, a))
#> <gradethis_graded_this_code: [Incorrect]
#>   I expected `a` where you wrote `b`. Give it another try.
#> >