generated at
Freerモナド
from Monad型クラス

Operationalモナドと全く同じものを指す
たぶん

https://captjakk.com/posts/2019-05-12-practical-intro-eff.html
https://qiita.com/dico_leque/items/d3fac511d0c284305c13
https://www.slideshare.net/konn/freer-monads-more-extensible-effects-59411772
https://qiita.com/YoshikuniJujo/items/686fedc92fd20ff70ab8
https://myuon-myon.hatenablog.com/entry/2017/04/29/213512