[samidalouche]

You basically consider the event log as a big collection,and you "fold over" the events in order to incrementally build your state/projection, the same way you would do with finite collections in a Functional language (scala, haskell, ...).

GetEventStore documentation has some examples of how you can create projections (https://geteventstore.com/blog/20130212/projections-1-theory...), which you can use as inspiration to build your own projections.