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  • janardhan bonu thinks this is interesting:

This is an important departure from functional programming: in a fold operation, the aggregation function g is a function of the intermediate value and the next item in the list—which means that values can be lazily generated and aggregation can begin as soon as values are available. In contrast, the reducer in MapReduce receives all values associated with the same key at once.

From

Cover of Data-Intensive Text Processing with MapReduce

Note

difference between Reduce and Fold