How does this work?
The TemplateHaskell code in Main.hs looks for any functions of the form dayXXY :: s -> t such that:
XXis a two-digit number from 01 to 25;Yis eitheraorb;- There is an instance of
From String sin scope; - There is an instance of
From t Stringin scope.
You can define instances for From String s and From t String however you like; the former represents your parser for that program and the latter how you want to display the result. For simple types (String, Text) implementations will exist; for others (Int, Integer) it's obvious (read/show).
Compiling the program will build an executable called aoc which can be called like:
# To run part a of day 1
cabal run aoc -- 1a
# To run both parts of day 20
cabal run aoc -- 20It will tell you at runtime if you try to run a part that hasn't been written in the program yet. It will automatically run the file input/dayXX through your From instance, run the dayXX part(s), run the From instance and output the resulting string.
Have fun!