two-fer, hexadecimal, nucleotide-codons: Removed the deprecated exercises

[ZapAnton]

As per this comment

[coriolinus]

Thanks for doing this!

Looking at check-exercises.sh, it appears that that we didn't anticipate ever deleting exercises. As such, I don't expect Travis to pass. A manual check of this branch with the command below completed successfully.

for ex in exercises/*; do
   (
     cd "$ex"
     ../../bin/test-exercise
   )
done

[coriolinus]

I plan to merge on Wednesday if nobody objects. I do not object if other maintainers merge this sooner.

[petertseng]

Wow, configlet lint might still expect even deprecated exercises to exist as long as they are listed in config.json. Now, note that for deprecated exercises:

• don't show up anywhere on the website if I have not already completed them
• do show up if I already completed them
• can still be fetched by explicitly specifying their ID

Since removing their implementations would cause configlet lint to fail, perhaps they will also have to be removed from there as well. I can't predict the effects of this

[coriolinus]

Travis fails due to configlet lint:

0.28s$ ./bin/configlet lint .
-> An exercise with slug 'nucleotide-codons' is referenced in config.json, but no implementation was found.
-> An exercise with slug 'two-fer' is referenced in config.json, but no implementation was found.
-> An exercise with slug 'hexadecimal' is referenced in config.json, but no implementation was found.
The command "./bin/configlet lint ." exited with 1.

While we could fix this by removing the deprecated exercises from config.json, I believe there to be value in retaining the deprecated exercises in config.json: this lets the maintainers see quickly in the future that these exercises used to be in the track, and were removed. I have filed a configlet issue requesting lint changes for this case.

I will therefore override Travis and merge this (finally!) anyway.

[petertseng]

Regarding whether it is safe to remove the code for deprecated exercises: My read is that it is safe. I show my work:

One may use the search "add back deleted, deprecated exercises"[1] to see the following quote
[1] https://github.com/search?q=org%3Aexercism+%22Add+back+deleted%2C+deprecated+exercises%22&type=Issues

When migrating the data from v1 to v2, we will lose all the solutions submitted to deprecated exercises, unless they exist in the track configuration.

We can delete them from the track again after we launch v2, if we want to.

(emphasis mine)

Note that git log -p -S deprecated shows that the most recent deprecation is June 2018, so only students who have been around for a long time will have seen the deprecated exercises. This means impact should be small.

The only value in potentially doing so is if a student can choose to still download it and complete anyway, but we have better alternatives for them:

• nucleotide-codons -> protein-translation
• hexadecimal -> all-your-base
• two-fer -> any exercise that actually does Option properly

[coriolinus]

Per new documentation in exercism/docs#126, this was not actually a safe change. I'd also believed that it was safe, but it's proven to be a problem. I will revert this.