[PL-sem-jr] Exercise from the 12/03 meeting

[Artem Pelenitsyn]

Hello juniors,

Attached is the file we worked out today. We have reminded ourself about
the maybe monad from the last time (evalChecked) and moved on with the
State monad (evalWithCount'). On the way, we discovered the Writer monad
(evalWithCount''). For that we even had to refresh out kindergarten algebra
class and recall what `m o n o i d s` are — so much fun!

Next time, we review Hugo's m03_donotation.hs and get back on track with
Tackling The Awkward squad paper. The latter tries to show us that monads
are not only useful for refactoring of pure functional programs, like the
ones we dealt with the last two sessions. But also, they can frame the
interface to the impure world with I/O, concurrency and FFI.

--
Best, Artem
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