再帰型
https://ja.wikipedia.org/wiki/再帰データ型
data List a = Nil | Cons a (List a)
同系再帰型
isorecursive type
https://en.wikipedia.org/wiki/Recursive_data_type#Isorecursive_types
https://keigoi.hatenadiary.org/entry/20091119/1258645209
等価再帰型
equirecursive type
https://en.wikipedia.org/wiki/Recursive_data_type#Equirecursive_types
https://keigoi.hatenadiary.org/entry/20121217/ocaml_equirecursive
/mrsekut-book-4274069117/234 (第20章 再帰型)