subsequencesを紐解く - Qiita

-- | The 'subsequences' function returns the list of all subsequences of the argument. -- -- > subsequences "abc" == ["","a","b","ab","c","ac","bc","abc"] subsequences :: [a] -> [[a]] subsequences xs = [] : nonEmptySubsequences xs -- | The 'nonEmptySubsequences' function returns the list of all subsequences of the argument, -- except for the empty list. -- -- > nonEmptySubsequences "abc" == ["a","