7607ddcc_All_About_Monads___HaskellWiki__Combination_strategy_for____
[Monad] Writer [Type of computation] Computations which write data in addition to computing values [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.
All About Monads - HaskellWiki