Blogged by Ujihisa. Standard methods of programming and thoughts including Clojure, Vim, LLVM, Haskell, Ruby and Mathematics written by a Japanese programmer. github/ujihisa The documentations about Array#<=> are: http://www.ruby-doc.org/core/classes/Array.html#M002227 (in English) http://doc.okkez.net/191/view/method/Array/i/=3c=3d=3e (in Japanese) For example: [1, 2, 3, 4] <=> [1, 2, 3, 3] #=> 1