output-based exercise

[jrvidal]

I've been wondering whether it'd be useful to have a 3rd kind of exercise, "output"/Mode::Output, that compares the stdout of an exercise to an expected value before moving on to the next one. This would imply a minor re-invention the #[test] framework, since we'd need to produce our own Expected/Actual output and stuff.

However, I feel that it would be a good fit for some println-based exercises, where the user can simply manipulate the program until it generates the desired output in a way that feels more natural to me than using #[test]. For instance, a hypothetical 0-exercise with an empty main() that expects you to print "Hello world". Or some of the if-based exercises, where the user can test branches until the correct one is executed. In fact, I suspect it'd be a good fit for most of the exercises for which we get complaints about them passing too soon.

@fmoko What do you think? If that sounds OK I'd be glad to send a patch.

[shadows-withal]

@jrvidal That sounds good! It's probably a good replacement for #[test]-based exercises, except for the ones that teach you about #[test].