[v3] Add prerequisites to Practice Exercises

[angelikatyborska]

closes #567 , closes #457

To launch v3, all practice exercises need to define which concepts are necessary for solving the exercise. The "topics" key becomes obsolete. Two new keys are added:

• prerequisites - a list of concepts that they students need to learn before they are able to solve the exercise well. It doesn't mean that they need to use all of those concepts while solving the exercise, but they need to know them to be able to choose the right one for the job. This key will be used to block students from solving an exercise too early.
• practices - one or two concepts on which this exercise focuses. Those concepts should be used to solve this exercise well. This key will be used to suggest to users which exercises to do in order to practice a concept they're interested in.

Ideally every concept would have at least one exercise that practices it.

Approach

When assigning concepts to exercises, I used this approach:

• Analyze the example solution, mentoring notes, and maybe some popular community solutions (not marked as old) for each exercise.
• A long prerequisites list can be used to steer students away from difficult exercises too early.
• If there are two or more equally valid approaches to a specific problems, all concepts necessary to use those multiple approaches are considered prerequisites (e.g. recursion vs enum, case vs multiple-clause-fuctions, agent vs genserver). For this reason, a lot of exercises list cond, case, if, multiple-clause-functions, pattern-matching and so on as necessary concepts, when only one or two of those is actually needed, because there is no single objective answer to which one is better :).
• If a more complex concept is a prerequisite for a given exercise, simpler concepts that are prerequisites to the more complex concept do not need to be listed. For example, if I need to know about erlang-libraries to solve an exercise, we don't need to explicitly list atoms or basics as a prerequisite. I only listed the very simple concepts like atoms, integers, or tuples for exercises where they played a bigger role.
• Use "topics" as a hint.
• Some of the concepts could be considered more "stylistic" than substantive, like the pipe operator and using module attribute as constants. I would avoid using those as prerequisites at all, except for the few exercises chosen to also list them as practices.
• Ideally every concept would be practiced by a few practice exercises.

During this process, it might become apparent that some concepts are missing. Adding, or planning for adding, new concepts is definitely possible, and even welcome! However, removing existing concepts or modifying them (changing their scope) might be difficult and hopefully we can avoid it.

Existing concepts

Concept tree: https://exercism.lol/tracks/elixir/concepts

slug	link 1	link 2
access-behaviour	introduction.md	about.md
agent	introduction.md	about.md
anonymous-functions	introduction.md	about.md
atoms	introduction.md	about.md
basics	introduction.md	about.md
binaries	introduction.md	about.md
bit-manipulation	introduction.md	about.md
bitstrings	introduction.md	about.md
booleans	introduction.md	about.md
case	introduction.md	about.md
charlists	introduction.md	about.md
closures	introduction.md	about.md
cond	introduction.md	about.md
dates-and-time	introduction.md	about.md
default-arguments	introduction.md	about.md
dynamic-dispatch	introduction.md	about.md
enum	introduction.md	about.md
erlang-libraries	introduction.md	about.md
errors	introduction.md	about.md
exceptions	introduction.md	about.md
file	introduction.md	about.md
floating-point-numbers	introduction.md	about.md
guards	introduction.md	about.md
if	introduction.md	about.md
integers	introduction.md	about.md
io	introduction.md	about.md
keyword-lists	introduction.md	about.md
list-comprehensions	introduction.md	about.md
lists	introduction.md	about.md
maps	introduction.md	about.md
module-attributes-as-constants	introduction.md	about.md
multiple-clause-functions	introduction.md	about.md
nil	introduction.md	about.md
pattern-matching	introduction.md	about.md
pids	introduction.md	about.md
pipe-operator	introduction.md	about.md
processes	introduction.md	about.md
protocols	introduction.md	about.md
randomness	introduction.md	about.md
ranges	introduction.md	about.md
recursion	introduction.md	about.md
regular-expressions	introduction.md	about.md
streams	introduction.md	about.md
strings	introduction.md	about.md
structs	introduction.md	about.md
tail-call-recursion	introduction.md	about.md
try-rescue	introduction.md	about.md
try-rescue-else-after	introduction.md	about.md
tuples	introduction.md	about.md

Planned concepts

For planned concepts, we can only add them as prerequisites after they were implemented. Their corresponding issues should list which practice exercises need them.

slug	link
genserver	issue
with	issue
task	issue
macros	issue

Exercise reference

slug	link 1	link 2	link 3
hello-world	example.ex	community solutions
rna-transcription	example.ex	community solutions	mentoring notes
word-count	example.ex	community solutions	mentoring notes
roman-numerals	example.ex	community solutions	mentoring notes
bob	example.ex	community solutions	mentoring notes
beer-song	example.ex	community solutions	mentoring notes
robot-simulator	example.ex	community solutions	mentoring notes
list-ops	example.ex	community solutions	mentoring notes
markdown	example.ex	community solutions
bank-account	example.ex	community solutions	mentoring notes
zipper	example.ex	community solutions
bowling	example.ex	community solutions
forth	example.ex	community solutions
resistor-color	example.ex	community solutions
two-fer	example.ex	community solutions
nucleotide-count	example.ex	community solutions
pig-latin	example.ex	community solutions
protein-translation	example.ex	community solutions
rotational-cipher	example.ex	community solutions
secret-handshake	example.ex	community solutions
space-age	example.ex	community solutions
strain	example.ex	community solutions
accumulate	example.ex	community solutions
acronym	example.ex	community solutions
raindrops	example.ex	community solutions
run-length-encoding	example.ex	community solutions
series	example.ex	community solutions
dnd-character	example.ex	community solutions
pangram	example.ex	community solutions
scrabble-score	example.ex	community solutions
sum-of-multiples	example.ex	community solutions
grade-school	example.ex	community solutions
isogram	example.ex	community solutions
twelve-days	example.ex	community solutions
collatz-conjecture	example.ex	community solutions
sublist	example.ex	community solutions
triangle	example.ex	community solutions
flatten-array	example.ex	community solutions
matching-brackets	example.ex	community solutions
anagram	example.ex	community solutions
hamming	example.ex	community solutions
matrix	example.ex	community solutions
phone-number	example.ex	community solutions
say	example.ex	community solutions
nth-prime	example.ex	community solutions
binary-search	example.ex	community solutions	mentoring notes
isbn-verifier	example.ex	community solutions
leap	example.ex	community solutions
tournament	example.ex	community solutions
all-your-base	example.ex	community solutions
binary-search-tree	example.ex	community solutions
grains	example.ex	community solutions
meetup	example.ex	community solutions
simple-linked-list	example.ex	community solutions
etl	example.ex	community solutions
binary	example.ex	community solutions
change	example.ex	community solutions
transpose	example.ex	community solutions
ocr-numbers	example.ex	community solutions
kindergarten-garden	example.ex	community solutions
diamond	example.ex	community solutions
luhn	example.ex	community solutions
scale-generator	example.ex	community solutions
rail-fence-cipher	example.ex	community solutions
gigasecond	example.ex	community solutions
hexadecimal	example.ex	community solutions
perfect-numbers	example.ex	community solutions
prime-factors	example.ex	community solutions
diffie-hellman	example.ex	community solutions
parallel-letter-frequency	example.ex	community solutions
palindrome-products	example.ex	community solutions
pascals-triangle	example.ex	community solutions
spiral-matrix	example.ex	community solutions
custom-set	example.ex	community solutions
saddle-points	example.ex	community solutions
connect	example.ex	community solutions
minesweeper	example.ex	community solutions
queen-attack	example.ex	community solutions
armstrong-numbers	example.ex	community solutions
simple-cipher	example.ex	community solutions
allergies	example.ex	community solutions
crypto-square	example.ex	community solutions
largest-series-product	example.ex	community solutions
clock	example.ex	community solutions
sieve	example.ex	community solutions
poker	example.ex	community solutions
atbash-cipher	example.ex	community solutions
grep	example.ex	community solutions
difference-of-squares	example.ex	community solutions
dominoes	example.ex	community solutions
alphametics	example.ex	community solutions
pythagorean-triplet	example.ex	community solutions
wordy	example.ex	community solutions
dot-dsl	example.ex	community solutions

[angelikatyborska]

I don't think protocols are necessary to solve this exercise, but the boilerplate code includes a protocol implementation (https://github.com/exercism/elixir/blob/main/exercises/practice/zipper/lib/bin_tree.ex#L15-L33) so I added it here to avoid students being confused by the boilerplate.

[neenjaw]

I think Zipper used to appear after Clock, so they were introduced to protocols by that point. I don't think students that I have encountered have been confused by this at this point with Clock having been removed from the core pathway.

[angelikatyborska]

A lot of solutions chose binary pattern-matching for splitting the string into three-letter substrings.

[angelikatyborska]

The example solution for this exercise uses metaprogramming like this:
https://github.com/exercism/elixir/blob/main/exercises/practice/protein-translation/.meta/example.ex#L49-L52

I wonder if this should become a concepts of its own? How would it be called, which parts of metaprogramming would it cover?

[angelikatyborska]

Maybe it could cover quote and unquote so that those are not new concepts when macros are introduced?

[angelikatyborska]

I'll keep this PR open, waiting for feedback, for at least 2 more weeks (28.02.2021).

[neenjaw]

Prettier attacked your formatting. 😬

I added a few here and there. I also added task to parallel-letter-frequencies and macros to dot-dsl. I think non-existent concepts are ignored by the system so that should be safe, but maybe that needs a confirm from @iHiD / @ErikSchierboom.

I think for the most part, most concepts are covered somewhere, I don't think there is a perfect coverage to have everything covered evenly and maybe once we can see it visually that one concept is disproportionately high/low we can make further adjustments.

[frerich]

It may be that I misunderstood the structure of this file -- but it seems to me that there is no such concept as macros (yet?).

[neenjaw]

not yet, but there should be. @angelikatyborska had a list of concepts that likely should be implemented at some point in one of her comments ☝🏻 up there. Concepts that don't exist, shouldn't affect things (afaik), but then this way we can be planning for their hopeful addition someday

[angelikatyborska]

We'll see what Jeremy or Erik say about this. I think I would personally prefer configlet to trip over non-existing concepts in prerequisites to warn me about typos. If we have to remove the non-existing concepts, they already have corresponding Github issues that list to which exercises it needs to be added.

[ErikSchierboom]

It is definitely good to list concepts that are not there yet, as the v3 website will just ignore them but you'll have them defined for when the concept is added later. configlet should indeed warn about using concepts in Practice Exercises that are not yet defined. For Concept Exercises, I feel like it should error if the concept is not specified.

[angelikatyborska]

I will merge this PR on 01.03.2021, in 3 days.

[iHiD]

I will merge this PR on 01.03.2021, in 3 days.

EXCITING.