589ea459_All_About_Monads___HaskellWiki__Type_of_computation
[Monad] Writer [Combination strategy for >>=] Written data is maintained separately from values. The bound function is applied to the input value and anything it writes is appended to the write data stream. [Type of computation] 
Computations which write data in addition to computing values
All About Monads - HaskellWiki
Type of computation