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What is so powerful about flatmap that it deserves such a place in the Scala folklore?
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In Scala, flatMap() method is identical to the map() method, but the only difference is that in flatMap the inner grouping of an item is removed and a sequence is generated. It can be defined as a blend of map method and flatten method.
It uses One-To-One mapping. It's mapper function produces multiple values (stream of values) for each input value. It's mapper function produces single values for each input value. Use the flatMap() method when the mapper function is producing multiple values for each input value.
The story I heard was that two preeminent Scala programmers were pairing when one of them started writing some code like this:
option match { case Some ... At which point the other said "What is this? Amateur hour? Flat map that shit!"
As to what's so powerful about flatMap, well... First, it's the fundamental monadic operator. That means it is a common operation shared by, for example, containers (such as Option, collections, etc), continuations, state, etc. Second, while you can de-construct an Option, that, as opposed to flatMap, is not a monadic operation, so it cannot be as widely applied. Also, it requires too much knowledge about the data you are manipulating.
Note: previously I said matching was slower than flatMap -- the opposite is true as a matter of fact, up to the most recent version of Scala at the time of this writing, 2.10.1.)
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