モナド則は、returnと(>>=)で>>59みたいに書くほかにreturnと(<=<)でも書ける

[1] return <=< x == x
[2] x <=< return == x
[3] (a <=< b) <=< c == a <=< (b <=< c)
[4] (a <=< b) . f = a <=< (b . f)

どっちがシンプルに思えるかは人に依るかも