swirl 2.0

• Uses addTaskCallback() as a mechanism to capture user input directly from the R prompt.
• During instruction, info() brings up a menu of options including bye(), skip(), play(), and nxt().
• skip() allows the user to skip the current question. swirl automatically evaluates the correct answer in the user's workspace in case future questions depend on the result.
• Includes a library of answer tests based on testthat, an R package designed by Hadley Wickham for unit testing.
• Tests user responses for correctness based various combinations of the aforementioned answer tests. A user is judged to have answered a question correctly when the answer tests specified for that question are satisfied.
• The answer tests operate on the R expression entered by the user, as opposed to the string representation of it (see swirl 1.0). This avoids marking a user incorrect for stylistic discrepancies such as including single spaces between function arguments, etc.
• Makes heavy use of R's S3 object oriented programming dialect to promote an easily extensible architecture. Incorporating new functionality simply requires writing new methods for existing "core" functions.
• Instructors can now author content in an R Markdown (.Rmd) file, then use rmd2csv() to create the CSV file from which swirl reads content. This is an experimental feature based on syntax employed by slidify, an R package designed by Ramnath Vaidyanathan for creating interactive web presentations.