[Haskell-beginners] histogram over large data

[Stephen Tetley]

It's likely the while loop can be replaced with a fold which has a Map
(or IntMap if the key is an Int) as the accumulator. For a large Map
you will need to pay attention to inserting elements strictly...

The more complicated bit is without any extra knowledge about the
implementation of the "database" `get_next_from_db()` is currently
magic.